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THE
ELEMENTS
OF
EXPERIMENTAL CHEMISTRY,
BY
WILLIAM HENRY, M.D. F.R.S.
Vice-Pres. of the Lit. and Phil. Soc. at Manchester; Member of the Roy. Med. and Wernerian Societies at Edinburgh; the Medico-Chirurgical and Geological Societies of London ; the Physical Soc. of Jena ; the Nat. Hist. Soc. of Moscow, &c.
THE EIGHTH EDITION,
COMPREHENDING ALL THE RECENT DISCOVERIES ; AND ILLUSTRATED
WITH NINE PLATES BY LOWRY.
VOL. I.
LONDON:
PRINTED FOR BALDWIN, CRADOCK, AND JOY, 47, PATERNOSTER-ROW?
AND R. HUNTER, SUCCESSOR TO JOHNSON, st. Paul’s church yard.
1818
historical j \ MEDICAL /
L. Baldwin, Printer, Kevv Bridge-street, London
TO
MR. JOHN DALTON,
President of the Lit. and Phil. Soc. of Manchester; Member of tiie Academy of Sciences of the Royal Institute of France ; &c.
AS A
TESTIMONY OF RESPECT
FOR TUB
ZEAL, DISINTERESTEDNESS, AND SUCCESS,
WITH WHICH
HE HAS DEVOTED HIMSELF TO THE ADVANCEMENT OF
CHEMICAL PHILOSOPHY,
THIS WORK IS INSCRIBED,
BY HIS FRIEND
THE AUTHOR.
Manchester ,
Oct. 1318.
ADVERTISEMENT
TO THE
EIGHTH EDITION,
DURING the interval which has elapsed since the pub¬ lication of the last edition of this work, the progress of Chemistry, though not distinguished by essential changes in the general principles of the science, has nevertheless been marked, not only by beneficial applications of those principles to the useful arts, but by the discovery of a great number of important facts, and of some new and interesting bodies. Among practical inventions, the Safety Lamp of Sir Hum¬ phry Davy stands pre-eminent, as a contribution from science to the interests of humanity, not resulting from accident, but suggested by general reasoning, and perfected by an admirable train of philosophical induction. — To our knowledge of indi¬ vidual bodies has been added that of a new alkali, a new earth, and two new metals; of a gas which, like chlorine, becomes acidified by union with hydrogen ; of new acids, composed of oxygen in combination with chlorine, with ni¬ trogen, and with phosphorus ; and of compounds, before un¬ discovered, derived from the vegetable and animal kingdoms. In a variety of instances, the properties of bodies, that had been long known, have been better ascertained, and more extensively investigated. Such additional evidence, too, of the nature of chlorine has arisen out of the further contro¬ versy respecting it, as to have satisfied me of the propriety of a change in its classification. It has been necessary, there¬ fore, again to revise the whole work with the greatest care ; to
VI
ADVERTISEMENT.
make considerable additions to many of the sections ; and to introduce a few entirely new ones. In a chapter of addenda, also, at the close of the second volume, the history of disco¬ veries will be found continued to the latest period which the publication would admit. To gain room for these improve¬ ments, without much enlarging the bulk of the volumes, I have rejected every thing which recent experience has corrected or rendered doubtful.
Though no pains have been spared to render the work a faithful abstract of the present state of Chemistry, yet it is not improbable that errors and omissions may still be disco¬ vered in it. In rectifying these, I hope to be assisted by a continuance of those candid criticisms, both through public and private channels of communication, to which I have al¬ ready been greatly indebted.
Manchester ,
Oct . 1818.
CONTENTS OF VOL. I.
/
Page
Introduction . . v
PART I.
An arranged Series oe Experiments and Processes
TO BE PERFORMED BY THE STUDENT OF CHEMISTRY.
CHAP. I. Of a Chemical Laboratory and Appa¬ ratus. . . 1
CHAP. II. Of Chemical Affinity . 14
Sect. I. Of Cohesion , Solution , and Crystalli¬ zation. . . . . . 15
II. Of Chemical Affinity , and the general
Phenomena of Chemical Action. ... 24?
III. Of the Proportions in which Bodies
combine ; and of the Atomic Theory 28
IV. Of Elective Affinity . . . . . 38
V. Of the Causes , which modify the Action
of Chemical Affinity . 40
VI. Of the Estimation of the Forces of
Affinity . . . 50
VII. Of Complex Affinity . . 53
VIII. Experimental Illustrations of Chemical
Affinity , Solution , fyc . 58
CHAP. III. Of Heat or Caloric . 64
Sect. I. General Observations on Heat . . ibid.
II. Illustrations of the Effects of Free
Caloric . . 72
III. Caloric the Cause of Fluidity . . . 91
IV . — — — Vapour ........ 96
CONTENTS.
vm
Page
CHAP. III. Sect. V. Specific Caloric . . . . 109
CHAP. IV. Of Light . . . . . . . . . . 112
CHAP. V9 Of Gases. . . . . 119
Sect. I. Of the Apparatus for Gases . ibid.
Classification of' Gases . 130
II. Oxygen Gas. . . . . . . 135
III. Chlorine Gas . . 142
IV. Niti 'ogen or Azotic Gas . 144
V. Atmospheric Air . 148
VI. Hydrogen Gas . . 154
CHAP. VI. Of the Composition, Decomposition, and
Properties of Water . 166
Sect. I. Synthesis, or Composition of Water. . ibid,
II. Analysis, or Decomposition of Water . 171
III. Properties and Effects of Water .... 174
CHAP. VII. On the Chemical Agencies of Common
and Galvanic Electricity . 183
Sect. I. Of the Construction of Galvanic Ar¬ rangements . 184
II. On the mutual Relation of Electricity
and Galvanism . 191
III. On the Chemical Agencies of Electri¬
city and Galvanism . 193
IV. Theory of the Changes produced by
Galvanic Electricity . 202
V. Theory of the Action of the Galvanic
v Pile . 205
CHAP. VIII, Alkalies. Their General Qualities. . 212
Sect. I. Pure Potash and PureSoda . ibid.
Art. 1. Their preparation and gene¬ ral Qualities . 212
Hydrated Alkalies ........ 213
2. Analysis of the two fixed Al¬
kalies. . . 216
3. Potassium.. . 212
Potassureted Hydrogen Gas . 229
4. Sodium . . 230
II. Lithia, or Lithina . . 233
III. Pure Ammonia . . 236
CHAP. VIII, Sect. III. Art. 1. Preparation and Qualities
of Ammonia ........ 236
2. Electrical Analysis of
Ammonia . . 240
3. On the Presence of Oxy¬
gen in Ammonia ; and on the Amalgam of Mercury and Ammonia 243
4. Action of Potassium on
Ammonia . . 246
CHAP. IX. Earths . . . . . . 249
Sect. I. Barytes . 252
II. Strontites „ . 255
HI. Lime . 257
IV. Magnesia . 260
V. Silex . 261
VI. Alumine . 265
VII. Zircon . . . 268
*VI II. Glucine . . 269
IX. Yttria, or Ittria . 270
X. Tkorina . 272
CHAP. X. Of Acids in general . 275
CHAP. XI. Carbonic Acid and its Base.- — Car¬
bonates. — Binary Compounds of
Carbon . 283
Sect. I. Carbon and Charcoal . ibid.
II. Combustion of Carbon . 287
III. Carbonic Acid . 290
IV. Carbonates ..................... 299
Art . 1. Sub-carbonate and Carbon¬ ate of potash . ibid.
2. Carbonate of Soda . . 304
3. Sub-carbonate and Bi-carbo¬
nate of Ammonia ...... 305
4. Carbonate of Barytes ..... 309
5. . — . Strontites.... 311
6. * - — - — — Lime. ....... 312
7. — - Magnesia .... 315
8. — - -* - Glucine . 316
V. Gaseous Oxide of Carbon , or Car¬
bon oris Oxide. ................. ibid.
X
CONTENTS.
Page
CHAP. XI, Sect. VI. Combination of Carbon with Hydro¬ gen , forming Carbureted. Hydro¬ gen Gas , or Hydro- Carburet .... SI 9 On the Fire-Damp of Coal Mines , and the Construction and Principle of the Safety Lamp , of Sir H. Davy 324 VII. Carburet of Hydrogen , or Cyanogen 327 CHAP. XII. Sulphur, —Sulphuric Acid,— Sulphates,
— Binary Compounds oe Sulphur . . . 323
Sect. I. Sulphur . . . . „ . . „ ibidm
II. Sulphuric Acid . . 333
III. Sulphurous Acid Gas . . . 341
IV. Combination of Sulphuric Acid with
Alkalies . . 344
Art. 1. SVphate of Potash . ibid .
2. - - — Seda ........ 346
3. — - — Ammonia .... 347
4. - — - Barytes . . 348
5. - - — . — — Strontites .... 351
«■
6. - - — — - — Lime. ....... 352
7. - — - Magnesia .... 353
8. - — — — - - Alumina .... 355
9. . — _ — _____ Glucine . 358
10. Sulphate of Zircon . ibid.
11. — - — - Yttria . ibid.
V. Sulphites . 359
VI. Binary Compounds of Sulphur. — ■
1 sty with Alkalies 2t/, with Hy¬ drogen . . 362
Art. 1. Sulphurets . . . . . ibid.
2. Sulphureted Hydrogen Gas 365
3. Hydro-Sulphurets . 369
4. Super-Sulphureted Hydro¬
gen, and Hydroguretted
Sulphurets . 371
Sulphuret of Carbon, or Al¬ cohol of Sulphur . 37 5
CHAPo XIII. Combination of Nitrogen with Oxygen,
constituting Nitric Acid, — Nitrous Gas, — Nitrous Oxide,— and Compounds of Nitric Acid with Alkalies . 379
Page
CHAP, XIIL Sect, I. Nitric Acid . . . , . 383
II. Nitrous Gas, or Nitric Oxide . 390
III. Gaseous Oxide of Nitrogen— -Nitrous
Oxide of Davy . . . s 398
IV, Nitrous Acid . . . 403
V. Per-nitrous Acid . ...» 405
VI. Nitrates . . 406
Art, 1. Nitrate of Potash. ......... ibid.
2. , , - Soda . . 413
3. . — ... — — — Ammonia . ibid.
4. - Barytes ......... 414
5„ — - Strontites ....... 415
5. . . . Lime ........... ibid.
7. „ — — _ Magnesia . . . 416
8. — Alumine ........ 417
9. Glucine. . . ibid.
10. — — — — Zircon .......... ibid.
11. . . . — - Yttria .......... 417
VII. Nitrites . . 418
CHAP, XIV. Muriatic Acid,— Oxymuriatic Acid, or
*
Chlorine,— and their Compounds .... 419
Muriatic Acid. . . . „. . . ibid.
Sect. I. Compound of Chlorine with Hydrogen 42 i II. Compound of Chlorine with Oxygen ,
— Oxides of Chlorine , — Chloric Acid, — Per-ch loric Acid ......... 431
Chlorine with Oxygen, Euchlorine ibid. Per-oxide of Chlorine ........ 4S2
Chloric Acid . . 433
III. Chlorine with Nitrogen. ........... 537
IV . Chlorine with the Metals of the Alka¬
lies and Earths, and with the Oxides
of these Metals . . 438
V . Chlorine with Charcoal, Carbonic Ox¬ ide, and Carbureted Hydrogen . . . 439
VI. Chlorine with Sulphur and its Com¬ pounds . . . . 441
VII. Chlorine with the Metals . . ibid.
Nomenclature of the Compounds of Muriatic and Oxymuriatic Acids . . . . . 442
CONTENTS.
xh
Page
CHAP. XIV. Sect. VIII. Muriates {Hydro-Chlorates) . 444
Art 1. Muriate of Potash . . ibid.
2. - — - Soda . 445
3. Ammonia .... 447
4. - - — Barytes . 449
5. — - Strontites .... 450
6. - • Lime . 451
7. - — - Magnesia . 453
8. — — — — — Alumine ..... ibid.
9. _ — — — Glucine . ibid.
10. ■■■ , ' i Zircon . . ibid.
11. Muriate of Yttria . 454
IX. Chlorates or Hyper -oxy -Muriates . . ibid. Art. 1. Chlorate or Hyper-oxy-
Muriate of Potash. ..... ibid •
2. Chlorate of Soda . 459
3. — ■■ Ammonia. . . . ibid.
4. Chlorates with earthy Bases. 460 (1.) Chlorate of Barytes . . ibid.
{2.) — — - Strontites. 461
(3.) - ■ Lime. . . . ibid.
X. Nitro-Muriatic Acid . 462
XI. Murio- Sulphuric Acid, . . 463
APPENDIX.
Description of the Plates . . . 465
INTRODUCTION *.
It has so long been a custom to preface a course of lectures with the history of the science which is their subject, that it may be necessary to state, briefly, the reasons that have in¬ duced me to depart from this established usage.
The history of chemistry may either be merely a history of the science, that is, a view of the progressive development of the facts and doctrines of which the science is composed ; or it may comprehend, also, the biography of chemists. The detail of the progress of discovery,' however, concerning par¬ ticular objects of chemical research, would certainly be pre¬ mature, at a period, when the student may be supposed to be ignorant of the external forms, and even of the existence, of no inconsiderable part of them. Respecting chemists them¬ selves, little can be said that can contribute to information or amusement ; for their lives, devoted to the abstract pursuits of science, have seldom been productive of events, that are suited to awaken or gratify general curiosity. Our interest, indeed, respecting philosophers, is seldom excited, unless by a knowledge of the additions which they have made to the
* The following discourse formed, originally, the introduction to a series of lectures delivered in Manchester, and was afterwards published under the title of “ A General View of the Nature and Objects of Chemistry, and of its Application to Arts and Manufactures As the readers of an elementary book may be presumed to require a similar plan of instruction, with the hearers of a popular course of lectures, I have thought it unneces¬ sary to alter the form under which the essay first appeared, though a few passages are applicable chiefly to the persons to whom it was originally ad¬ dressed.
XIV
INTRODUCTION.
facts or theories of a science ; and with these a lecturer rnav fairly presume, however the fact may really be, that his hearers, at the commencement of a course, are wholly un¬ acquainted. On these grounds, therefore, I hope to be ex¬ cused for devoting to other purposes the time, that would have been allotted to the history of the science. For this, will be substituted a brief view of the nature and objects of
chemistry ; of its connexion with the arts and with other
(
sciences ; and an outline of the plan on which the following lectures will be conducted.
Natural philosophy, in its most extensive sense, is a term comprehending every science, that has for its objects the pro¬ perties and affections of matter. But it has attained, by the sanction of common language, a more limited signification ; and chemistry, though strictly a branch of natural philo¬ sophy, is generally regarded as a distinct science. Between the two it may, perhaps, be difficult to mark out precisely the line of separation : but, an obvious character of the facts of natural philosophy is, that they are always attended with sensible motion ; and the determination of the laws of motion is peculiarly the office of its cultivators. Chemical changes, on the other hand, of the most important kind, often take place without any apparent motion, either of the mass, or of it's minute parts ; and where the eye is unable to perceive that any change has occurred. The laws of gravitation, of cen¬ tral forces, and all the other powers that fall under the cogni¬ zance of the natural philosopher, produce, at most, only a change of place in the bodies that obey their influence. Biit, in chemical changes, we may always observe an important difference in the properties of things : their appearances and qualities are completely altered, and their individuality de¬ stroyed. Thus, two highly corrosive substances, by uniting chemically together, may become mild and harmless ; the combination of two colourless substances may present us with a compound of brilliant complexion ; and the union of two fluids, with a compact and solid mass.
INTRODUCTION.
XV
Chemistry, therefore, may be defined, that science, the ob¬ ject of which is to discover and explain the changes of com¬ position that occur among* the integrant and constituent parts of different bodies *.
From this definition, it may readily be conceived, how wide is the range of chemical inquiry ; and, by applying it to the various events that daily occur in the order of nature, we shall be enabled to separate them with accuracy, and to allot, to the sciences of natural philosophy and chemistry, the proper objects of the cultivation of each. Whenever a change of place is a necessary part of any event, we shall call in the aid of the former. When this condition may be dispensed with, we shall resort to chemistry for the light of its principles. But it will be often found, that the concurrence of the two sciences is essential to the full explanation ol phenomena. The water of the ocean, for example, is raised into the atmo¬ sphere by its chemical combination with the matter of heat ; but the clouds, that are thus formed, maintain their elevated situation by virtue of a specific gravity inferior to that of the lower regions of the air, — a law, the discovery and application of which are due to the natural philosopher, strictly so called.
It has not been unusual to consider chemistry, under the twofold view of a science and of an art. This arrangement,
O 7
however, appears to have had its origin in an imperfect dis¬ crimination between two objects, that are essentially distinct. Science consists of assemblages of facts, associated together in classes, according to circumstances of resemblance or analogy. The business of its cultivators is, first, to investigate and establish individual truths, either by the careful observation of natural appearances, or of new and artificial combinations of phenomena produced by the instruments of experiment. The next step is the induction, from well ascertained facts,
* The reader, who wishes to examine other definitions of chemistry, will find a variety of them, collected by Dr. Black, in the first volume of his a Lectures/7 published, since his death, by Professor Robison.
5
XVI
INTRODUCTION.
of general principles or laws, more or less comprehensive in- their extent, and serving, like the classes and orders of natural history, the purposes of an artifical arrangement. Of such a body of facts and doctrines, the science of chemistry is composed. But the employment of the artist consists merely in producing a given effect, for the most part by the sole guidance of practice or experience. In the repetition of pro¬ cesses, he has only to follow an established rule ; and, in the improvement of his art, he is benefited generally by fortuitous combinations, to which he has not been directed by any general axiom. An artist, indeed, of enlarged and en¬ lightened mind, may avail himself of general principles, and may employ them as an useful instrument in perfecting esta¬ blished operations : but the art and the science are still marked by a distinct boundary. In such hands, they are auxiliaries to eacli other; the one contributing a valuable accession of facts ; and the other, in return, imparting fixed and compre¬ hensive principles, which simplify the processes of art, and direct to new and important practices.
The possession of the general principles of chemistry en¬ ables us to comprehend the mutual relation of a great variety of events, that form a part of the established course of nature. It unfolds the most sublime views of the beauty and harmony of the universe ; and developes a plan of vast extent, and of uninterrupted order, which could have been conceived only by perfect wisdom, and executed by unbounded power. By withdrawing the mind, also, from pursuits and amusements that excite the imagination, its investigations may tend, in common with the rest of the physical sciences, to the improve¬ ment of our intellectual and moral habits ; to strengthen the faculty of patient and accurate thinking ; and to substitute placid trains of feeling, for those which are too apt to be awakened by the contending interests of men in society, or the imperfect government of our own passions.
The class of natural events that call for the explana¬ tion of chemical science, is of very considerable extent ; and
6
INTRODUCTION.
XVII
the natural philosopher (using this term in its common ac¬ ceptation) is wholly incompetent to unfold their connexion. He may explain, for example, on the principles of his own science, the annual and diurnal revolutions of the Earth, and part of the train of consequences depending on these rotations. But here he must stop ; and the chemist must trace the effects, on the Earth’s surface, of the caloric and light derived from the sun ; the absorption of caloric by the various bodies on which it falls ; the consequent fluidity of some, and volatiliza¬ tion of others ; the production of clouds, and their condensa¬ tion in the form of rain ; and the effects of this rain, as well as of the sun’s heat, on the animal, vegetable, and mineral king¬ doms. In these minuter changes, we shall find, there is not less excellence of contrivance, than in the stupendous move¬ ments of the planetary system. And they interest us even more nearly ; because, though not more connected with our existence or comfort, yet they are more within our sphere of observation ; and an acquaintance with their laws admits of a more direct application to human affairs.
There is another branch of knowledge (that of natural history), which is materially advanced by the application of chemical science. The classifications of the naturalist are derived from an examination and comparison of the external forms, both of animate and inanimate bodies. He distributes the whole range of nature into three great and comprehensive kingdoms, —the animal, the vegetable, and the mineral. Each of these, again, is subdivded into several less extensive classes ; and individual objects are referred to their place in the system, by the agreement of their characters, with those assigned to the class, order, and genus. In the different departments of natural history, these resemblances vary in distinctness, in facility of observation, and in certainty of description. Thus, the number and disposition of the parts of fructification in vegetables afford marks of discrimination, which are well defined, and easily ascertained. But minerals, that are not voh L b
XV11I
INTRODUCTION.
possessed of a regularly crystallized form, are distinguished by outward qualities that scarcely admit of being accurately conveyed by language ; such as minute shades of colour ; or trifling differences of hardness, transparency, See. To the evidence of these loose and varying characters, that of the chemical composition of minerals has within the few past years been added ; and mineralogy has been advanced, from a con¬ fused assemblage of its objects, to -the dignity of a well me¬ thodized and scientific system. In the example of crystal¬ lized bodies, the correspondence between external form and chemical composition, has been most successfully traced by the genius of Haiiy ; whose method of investigation has enabled him, in numerous instances, to anticipate, from phy¬ sical characters, the results of the most skilful and laborious analysis.
It is unnecessary to pursue this part of the subject to a greater extent ; because, to all who have been in the habit of philosophical investigation, the connexion between the sci¬ ences must be sufficiently apparent ; and because there is another ground, on which chemistry is more likely to claim, with success, the respect and attention of the great mass of mankind. This is, its capacity of ministering to our wants and our luxuries, and of instructing us to convert to the or¬ dinary purposes of life, many substances which nature pre¬ sents in a rude and useless form. The extraction of metals from their ores ; the conversion of the rudest materials into the beautiful fabrics of glass and porcelain ; the production of wine, ardent spirits, and vinegar; and the dyeing of linen, cotton, and woollen manufactures,— -are only a few of the arts that are dependent on chemistry for their improvement, and even for their successful practice.
It cannot, however, be denied, that all the arts which have been mentioned were practised in times when the rank of che¬ mistry, as a science, was extremely degraded ; and that they are the daily employment of unlettered and ignorant men.
INTRODUCTION.
NIX
But to what does this confession amount ? and how far does it prove the independence of the above arts on the science of chemistry ?
The skill of an artist is compounded of knowledge and of manual dexterity. The latter, it is obvious, no science can teach. But the acquirement of experience, in other words, a talent for the accurate observation of facts, and the habit of arranging facts in the best manner, may be greatly facilitated by the possession of scientific principles. Indeed, it is hardly possible for any one to frame rules for the practice of a che¬ mical art, or to profit by the rules of others, wrho is unac¬ quainted with the general doctrines of the science. For, in, all rules, it is implied, that the promised effect will only take place, when circumstances are precisely the same as in the case under which the rule was formed. To ensure an un¬ erring uniformity of result, the substances, employed in che¬ mical processes, must be of uniform composition and excel¬ lence ; or, when it is not possible to obtain them thus un¬ varied, the artist should be able to judge precisely of the defect, that he may proportion his agents according to their qualities. Were chemical knowledge more generally pos¬ sessed, we should hear less of failures and disappointments in chemical operations ; and the artist would commence his pro¬ ceedings, not, as at present, with distrust and uncertainty, but with a confident and well grounded expectation of success.
It will scarcely be contended, that any one of the arts has hitherto attained the extent of its possible perfection. In all, there is yet a wide scope for improvement, and an extensive range for ingenuity and invention. But from what class of men are we to expect useful discoveries ? Are we to trust, as hitherto, to the favour of chance and accident ; to the for¬ tuitous success of those who are not guided in their experi¬ ments by any general principles ? Or shall we not rather endeavour to inform the artist, and induce him to substitute, for vague and random conjecture, the torch of induction and
b 2
XX
INTRODUCTION.
of rational analogy? In the present imperfect state of his knowledge, the artist is even unable fully to avail himself of those fortunate accidents, by which improvements sometimes occur in his processes ; because, to the eye of common ob¬ servation, he may have acted agreeably to established rules, and have varied in circumstances which he can neither per¬ ceive nor appreciate. The man of science, in these instances, sees more deeply, and, by availing himself of a minute and accidental difference, contributes at once to the promotion of his own interest, and to the advancement of his art.
But it is the union of theory with practice that is now re¬ commended. And “ when theoretical knowledge and prac¬ tical skill are happily combined in the same person, the in¬ tellectual power of man appears in its full perfection, and equally fits him to conduct, with a masterly hand, the details of ordinary business, and to contend successfully with the un¬ tried difficulties of new and perplexing situations. In con¬ ducting the former, mere experience may frequently be a sufficient guide ; but experience and speculation must be com¬ bined to prepare us for the latter*.” “ Expert men,” says Lord Bacon, “ can execute and judge of particulars one by one ; but the general counsels, and the plots, and the mar¬ shalling of affairs, come best from those that are learned.”
This recommendation to artists, of the acquirement of sci¬ entific knowledge, is happily sanctioned by the illustrious success, in our own days, of the application of theory to the practice of certain arts. Few persons are ignorant of the benefits, that have resulted to the manufactures of this country, from the inventions of Mr. Watt and Mr. Wedgwood ; both of whom have been not less benefactors of philosophy than eminent for practical skill. The former, by a clear insight into the doctrine of latent heat, resulting, in a great measure, from his own acuteness and patience of investigation, and
* Stewart’s Elements of the Philosophy of the Human Mind, chap. ir. sect. 7.
INTRODUCTION,
XXI
seconded by an unusual share of mechanical skill, has per¬ haps brought the steam-engine to its acme of perfection, Mr. Wedgwood, aided by the possession of extensive chemical knowledge, made rapid advances in the improvement of the art of manufacturing porcelain ; and, besides raising himself to great opulence and distinction, has created for his country a source of most profitable and extensive industry. In an art, also, which is nearly connected with the manufactures of our own town, and the improvement of which must, there¬ fore, ce come home to our business and bosoms,” we owe un¬ speakable obligations to two speculative chemists, — to Scheele, who first discovered the oxygenized muriatic acid ; and to Berthollet, who first instructed us in its application to the art of bleaching.
Examples, however, may be urged against indulgence in theory ; and instances are not wanting, in which the love of speculative refinement has withdrawn men entirely from the straight path of useful industry, and led them on gradually to the ruin of their fortunes. But from such instances, it would be unfair to deduce a general condemnation of theo- retical knowledge. It would be the common error of arguing against things that are useful, from their occasional abuse.— In truth, projects which have, for their foundation, a depend¬ ence on chemical principles, may be undertaken with a more rational confidence, than such as have in view the accomplish¬ ment of mechanical purposes ; because, in chemistry, we are better able, than in mechanics, to predict, from an experi¬ ment on a small scale, the probable issue of more extensive attempts. No one, from the successful trial of a small ma¬ chine, can affirm, with unerring certainty, that the same suc¬ cess will attend one on a greatly enlarged plan ; because the amount of the resistances, that are opposed to motion, in¬ creases often in a ratio greater than, from theory, could ever have been foreseen : but the same law, by which the mineral alkali is extracted from a pound of common salt, must equally operate on a thousand times the quantity ; and, even when we
XXII
INTRODUCTION.
augment our quantities in this immense degree, the chemical affinities, by which so large a mass is decomposed, are exerted only between very small particles. The failures of the me- chanic, therefore, arise from the nature of things ; they occur, because he has not in his power the means of foreseeing and calculating the causes that produce them. But, if the chemist fail in perfecting an economical scheme on a large scale, it is either because he has not sufficiently ascertained his facts on a small one, or has rashly embarked in extensive speculations, without having previously ensured the accuracy of his esti¬ mates.
The benefits which we are entitled to expect from the efforts of the artist and the man of science, united in one person, and at the same time tempered and directed by prudential wisdom, affect not only individual but national prosperity. To the support of its distinction, as a commercial nation, this country is to look for the permanency of its riches, its power, and, perhaps, even of its liberties ; and this pre-eminence is to be maintained, not only by local advantages, but on the more certain ground of superiority in the productions of its arts. Impressed with a full conviction of this influence of the sciences, a neighbouring and rival people offer the most public and respectful incitements to the application of theory in the improvement of the chemical arts ; and, with the view of promoting this object, national institutions have been formed among them, which have been already, in several in¬ stances, attended with the most encouraging success. It may be sufficient, at present, to mention, as an example, that France, during a long war, supplied, from her own native resources, her enormous, and, perhaps, unequalled consump¬ tion of nitre.
The general uses of chemistry have been thus fully en¬ larged upon, because it is a conviction of the utility of the science, that can alone recommend it to attentive and per¬ severing study. It may now be proper to point out, in detail, a few of its more striking applications.
INTRODUCTION.
XX1U
I. The art which is, of all others, the most interesting, from its subserviency to wants that are interwoven with our nature, is agriculture, or the art of obtaining, from the earth, the largest crops of useful vegetables at the smallest expense.
The vegetable kingdom agrees with the animal one, in the possession of a living principle. Every individual of this kingdom is regularly organized, and requires for its support an unceasing supply of food, which is converted, as in the animal body, into substances of various forms and qualities. Each plant has its periods of growth, health, disease, decay, and death ; and is affected, in most of these particulars, by the varying condition of external circumstances. A perfect state of agricultural knowledge would require, therefore, not only a minute acquaintance with the structure and economy of vegetables, but with the nature and effects of the great variety of external agents, that contribute to their nutriment, or in¬ fluence their state of health and vigour. It can hardly be expected, that the former attainment will ever be generally made by practical farmers ; and it is in bringing the agricul¬ turist acquainted with the precise composition of soils and manures, that chemistry promises the most solid advantages. Indeed, any knowledge that can be acquired on this subject, without the aid of chemistry, must be vague and indistinct* and can neither enable its possessor to produce an intended effect with certainty, nor be communicated to others in lan» guage sufficiently intelligible. Thus wre are told, by Mr. Arthur Young, that, in some parts of England, any loose clay is called marl, in others marl is called chalk, and in others clay is called loam. From so confused an application of terms, all general benefits of experience in agriculture must be greatly limited.
Chemistry may, to agriculturists, become a universal lan¬ guage, in which the facts, that are observed in this art, may be so clothed, as to be intelligible to all ages and nations. It would be desirable, for example, when a writer speaks of clay, loam, or marl, that he should explain his conception of these terms, by stating the chemical composition of each substance
XXIV
INTRODUCTION.
expressed by them. For, all the variety of soils and manures, and all the diversified productions of the vegetable kingdom, are capable of being resolved, by chemical analysis, into a small number of elementary ingredients. The formation of a well defined language, expressing the proportion of these elements in the various soils and manures, now so vaguely characterized, would give an accuracy and precision, hitherto unknown, to the experience of the tillers of the earth.
It has been said, by those who contend for pure empiricism in the art of agriculture, that it has remained stationary, notwithstanding all improvements in the sciences, for more than two thousand years. (6 To refute this assertion,” says Mr. Kirwan, 66 we need only compare the writings of Cato, Columella, or Pliny, with many modern tracts, or still better, with the modern practice of our best farmers.” — “ If the exact connection of effects with their causes,” he adds, “ has not been so fully and extensively traced in this as in other subjects, we must attribute it to the peculiar difficulty of the investigation. In other subjects, exposed to the joint oper¬ ation of many causes, the effect of each, singly and exclusively taken, may be particularly examined, and the experimenter may work in his laboratory, with the object always in his view*. But the secret processes of vegetation take place in the dark, exposed to the various and indeterminable influences of the atmosphere, and require, at least, half a year for their completion. Hence the difficulty of determining on what peculiar circumstance success or failure depends ; for, the diversified experience of many years can alone afford a ra¬ tional foundation for solid, specific conclusions*.”
II. To those who study medicine as a branch of general science, or with the more important view of practical utility, chemistry may be recommended with peculiar force and pro¬ priety. — The animal body may be regarded as a living ma¬ chine, obeying the same laws of motion as are daily exempli-
* See Kirwan on Manures.
INTRODUCTION.
XXV
fled in the productions of human art. The arteries are long, flexible, and elastic canals, admitting, in some measure, the application of the doctrine of hydraulics ; and the muscles are so many levers, of precisely the same effect with those which are employed to gain power in mechanical contrivances. But there is another view, in which, with equal justice, the living body may be contemplated. It is a laboratory, in which are constantly going forward processes of various kinds, dependent on the operation of chemical affinities. The con¬ version of the various kinds of food into blood, a fluid of com¬ paratively uniform composition and qualities ; the production of animal heat by the action of the air on that fluid, as it passes through the lungs ; and the changes, which the blood afterwards undergoes in its course through the body, — are all, exclusively, subjects of chemical inquiry. To these, and many other questions of physiology, chemistry has of late years been applied with the most encouraging success ; and it is to a long continued prosecution of the same plan, that we are to look for a system of physiological science, which shall derive new vigour and lustre from the passing series of years.
It must be acknowledged, however, as has been observed by Sir H. Davy*, that “ the connexion of chemistry with physiology has given rise to some visionary and seductive theories; yet even this circumstance has been useful to the public mind, in exciting it by doubt, and in leading it to new investigations. A reproach, to a certain degree just, has been thrown upon those doctrines known by the name of the chemical physiology; for, in the applications of them, spe¬ culative philosophers have been guided rather by the ana¬ logies of words than of facts. Instead of endeavouring slowly to lift up the veil, which conceals the wonderful phenomena of living nature; full of ardent imaginations, they have vainly and presumptuously attempted to tear it asunder.”
* In his excellent u Discourse, Introductory to a Course of Lectures/’ &c. London. Johnson. 1802.
XXVI
INTRODUCTION®
III. There is an extensive class of arts, forming, when viewed collectively, a great part of the objects of human industry, which do not, on a loose and hasty observation, present any general principle of dependency or connexion. But they appear thus disunited, because we have been accus¬ tomed to attend only to the productions of these arts, which are, in truth, subservient to widely different purposes. Who would conceive, for instance, that iron and common salt ; the one a metal, the use of which results from its hardness, duc¬ tility, and malleability ; the other a substance, chiefly valuable from its acting as a preservative and seasoner of food, — are furnished by arts alike dependent on the general principles of chemistry? The application of science, in discovering the principles of these arts, constitutes what has been termed economical chemistry ; amongst the numerous objects of which, the following stand most distinguished :
1st. Metallurgy , or the art of extracting metals from their ores, comprehending that of Assayings by which we are enabled to judge, from the composition of a small portion, of the propriety of working large and extensive strata. To the metallurgist, also, belong the various modifications of the metals when obtained, and the union of them together, in different proportions, so as to afford compounds adapted to particular uses. — Throughout the whole of this art, much practical knowledge may be suggested by attention to the general doctrines of chemistry. The artist may receive use¬ ful hints respecting the construction of furnaces for the fusion of ores and metals; the employment of the proper fluxes: the
utility of the admission or exclusion of air; and the con- *
version of the refuse of his several operations to useful pur¬ poses. When the metals have been separated from their ores, they are to be again subjected to various chemical pro¬ cesses. Cast or pig iron is to be changed into the forms of wrought or malleable iron and of steel. Copper, by com¬ bination with zinc or tin, affords the various compounds of brass, pinchbeck, bell-metal, gun-metal, &c. Even the art of printing owes something of its present unexampled perfection to the improvement of the metal of types.
INTRODUCTION. XXvii
2d. Chemistry is the foundation of those arts that furnish us with saline substances , an order of bodies highly useful in the business of common life. Among these, the most con¬ spicuous are, sugar in all its various forms; the vegetable and mineral alkalies, known in commerce by the names of potash, pearlasli, and barilla; common salt; green and blue vitriol, and alum ; nitre or saltpetre ; sugar of lead 5 borax ; and a long catalogue, which it is needless to extend farther.
3d. The manufacturer of glass, and of various kinds of pottery and porcelain , should be thoroughly acquainted with the nature of the substances he employs : with their fusibility, as affected by difference of proportion, or by the admixture of foreign ingredients ; with the means of regulating and mea¬ suring high degrees of heat; with the principles on which depend the hardness of his products, and their fitness for bearing the vicissitudes of heat and cold ; and with the che¬ mical properties of the best adapted colours and glazings.— Even the humble art of making bricks and tiles has received, from the chemical knowledge of Bergman, the addition of se¬ veral interesting facts.
4th. The preparation of various kinds of fermented liquors , of wine, and ardent spirits, is intimately connected with che¬ mical principles. Malting, the first step in the production of some of these liquors, consists in the conversion of part of the grain into saccharine matter, essential in most instances to the success of the fermentative change. To acquire a precise ac¬ quaintance with the circumstances, that favour or retard the process of fermentation, no small share of chemical know¬ ledge is required. The brewer should be able to ascertain, and to regulate exactly, the strength of his infusions, which will vary greatly when he has seemingly followed the same routine. He should be aware of the influence of minute changes of temperature in retarding or advancing ferment¬ ation ; of the means of promoting it by proper ferments ; and of the influence of the presence or exclusion of atmospherical air. A complete acquaintance with the chemical principles
XXV111
INTRODUCTION,
of his art, can hardly fail to afford him essential aid in its practice.
The production of ardent spirits is only a sequel of the vinous fermentation, and is, therefore, alike dependent on the doctrines of chemistry.
5th. The arts of bleaching , dyeing , and printing , are, throughout, a tissue of chemical operations. It is not unusual to hear the new mode of bleaching distinguished by the ap¬ pellation of the chemical method ; but it is, in truth, not more dependent on the principles of this science, than the one which it has superseded, nor than the kindred arts of dyeing and printing. In the instance of bleaching, the obligation due to the speculative chemist is universally felt and acknowledged. But the dyer and the printer have yet to receive from the phi¬ losopher some splendid invention, which shall command their respect, and excite their attention to chemical science. From purely speculative men, however, much less is to be expected, than from men of enlightened experience, who endeavour to discover the design and reason of each step in the processes of their arts, and fit themselves for more effectual observation of particular facts, by diligently possessing themselves of general truths.
The objects of inquiry that present themselves to the dyer and printer, are of considerable number and importance. The preparation of goods for the reception of colouring matter ; the application of the best bases, or means of fixing fugitive colours ; the improvement of colouring ingredients themselves ; and the means of rendering them permanent, so that they shall not be affected by soap, or by the accidental contact of acids or other corrosive bodies: are among: the subjects of chemical investigation. It is the business of the dyer, therefore, to become a chemist ; and he may be assured that, even if no brilliant discovery should be the reward of the acquisition, he will yet be better fitted by it for conducting common operations, with certain and unvaried success.
6 th. The tanning and preparation of leather are processes
INTRODUCTION.
xxix
strictly chemical, which were involved in mystery till they were reduced to well established principles by the researches of Sequin, and by the subsequent experiments of Davy. In this, as in most other examples, the application of science to the practical improvement of an art, has to encounter the b- stacles of ignorance and prejudice. But the interests of nmn. are sure finally to prevail ; and the most bigotted attachment established forms must give place to the clearly demonstrated utility of new practices. Such d ration is generally
furnished by some artist r‘r -.tore eoh iteiied views than his neighbours, who has the pirit to deviate from ordinary rules ; and thus becomes (not unfrequently with some personal sacri¬ fice) a model for the imitation of others, and an important benefactor of mankind.
Many other chemical arts might be enumerated ; but enough, I trust, has been said, to evince the connexion between practical skill and the possession of scientific knowledge. I shall now proceed to develope the plan, on which the fol¬ lowing course of instruction will be conducted.
There are two methods of delivering the general doctrines of chemistry, and the facts connected with them. The one consists in a historical detail of the gradual progress of the science; and, in pursuing this plan, we follow the natural progress of the human mind, ascending from particular facts to the establishment of general truths. But a strong objection to its adoption is, that we are thus led into a minuteness of detail, which is ill suited to the plan of elementary instruction. In the other mode of arrangement, we neglect wholly the order of time in which facts were discovered, and class them under general divisions so framed as to assist the mind in ap¬ prehending and retaining the almost infinite variety of parti¬ cular truths.
In a classification of the objects of chemistry, we may either begin writh those substances, which are deemed to be simple, and proceed gradually to the more complicated : — or we may take bodies, as they are usually presented to us, and arrange them according to the resemblances of their external
XXX
INTRODUCTION.
characters; making the development of their composition a subordinate part of the plan. To the former, or synthetic method, ther e is this strong objection, — that as we are proba¬ bly still very remote from a knowledge of the true elements of i fitter, it mst be liable, in the progress of science, to fre¬ quent and fundamental changes. It has been found necessary, for example, in* consequence of Sir H. Davy’s discoveries, to remove the fixed alkalies and the earths from the class of simple to that of comneund bodies. Besides, it may be urged, where are we to place uurse substances, which have hitherto resisted all attempts at their analysis, and yet have a striking resem¬ blance, in natural characters, to the bodies with which they are already associated ? For these reasons it appears to me, that one arrangement is preferable to another, on no other ground, than as it is better adapted for communicating a knowledge of the subject; for all must be equally remote from that perfection, which cannot be considered as attained, till the science of chemistry shall no longer be capable of im¬ provement.
The order, which I have adopted as most eligible, is to commence with those facts, which lead most directly to the establishment of general principles. Attraction or affinity, as the great cause of all chemical changes, and as admitting of illustration by phenomena that are sufficiently familiar, has a primary claim to consideration. Next to that of attraction, the influence of Heat, over the forms and properties of bodies, is the most generally observed fact ; and as heat is a power, which is constantly opposed to that of affinity, there is the more propriety in contrasting their operation. With heat, Light also, as a repulsive agent, is frequently associated, and Electricity belongs to the same class of powers. But as the action of electricity consists, chiefly, in effecting the disunion of chemical compounds, I have removed it from that place in the system, which seems naturally to belong to it. For before we can understand the general laws of electrochemical agency, it is necessary to know something of oxygen and a few of the inflammable bodies ; nor can the theory of the excitation of
INTRODUCTION.
XXXI
galvanic electricity be made at all intelligible, without this previous knowledge.
The phenomena of heat, and the laws deduced from them, conduct us naturally to the great source of that fluid, which will be traced to a class of bodies agreeing, in mechanical properties, writh the air of our atmosphere, and called airs or gases. These gases, we shall find, consist partly of gravitating matter, and partly of an extremely subtile fluid, which im¬ presses on our organs the sensation of heat, and is called caloric When the ponderable ingredients, usually called the bases , of these gases, combine together, or with other bodies, caloric is given out, and newr compounds are generated. It is on the possession or absence of the property of decom¬ posing one of them, oxygen gas, that a comprehensive division has been made of bodies into combustible and incombustible . In this view of the subject, combustion necessarily implies the fixation of oxygen ; but the term has lately been extended to every case of energetic chemical combination, which is accom¬ panied with heat and light. With oxygen, chlorine possesses such numerous and close analogies, that it can only with pro¬ priety be placed along with that element, in the class of chemical agents, which have been called supporters of combust ion. Iodine is, also, entitled to the same rank ; and it is for purposes of convenience, and with the view of giving a more complete his¬ tory of it, that I have placed it in a different part of the work.
The next division of bodies, that claim our attention, in¬ cludes those, which are formed either by the mixture or union of the simple gases or of their bases. Thus oxygen and ni¬ trogen gases compose atmospheric air ; and hydrogen and oxygen, water. Nitrogen and hydrogen, by their union, afford ammonia ; and with this fluid the fixed alkalies are naturally associated. The detail of properties belonging to the alkalies and earths is, indeed, a necessary preliminary to that of the acids, the most important quality of which is, that they constitute, with the alkalies and earths, an extensive class of neutral salts. The consideration of the bases of the alkalies
Light and electricity are probably, also, constituents of the gases.
XXX11
INTRODUCTION.
and earths has been made to follow that of the bodies them¬ selves, because these bases are the products of refined and complicated operations, which could scarcely have been other¬ wise understood. The fixed alkalies, also, precede the volatile ones, on account of the singular effects of potassium on am¬ monia.
The next class of compounds is that of Acids. With each of these I have connected the history of its base, when known ; for as several of these bodies have already lost, and others ap¬ pear likely to lose, their title to be considered as elementary, it becomes merely a question of convenience where they should be placed. In treating of the acids, their relation will be traced to those bodies only which have already been de¬ scribed ; for it would be unseasonable to detail their action on metals, till that class of substances has been specifically dis¬ cussed.
Having dismissed the consideration of such elementary bodies, as are distinguished by affording acids when combined with oxygen, of the properties of acids thus generated, and of the compounds afforded by the union of acids with alkalies ; an important division of elementary substances will next claim our attention, viz. the Metals.
The class of bodies, it is usual to introduce at a much earlier period: but I have adopted a different order, from the consideration, that, with the previous knowledge of the con¬ stitution and qualities of acids, the history of the metals may be made much more complete; and, especially, that all the various modes and phenomena of their combination with oxygen and chlorine may be more distinctly explained. The more complex productions of the vegetable and animal king¬ doms will be the last step in our progress through the chemical arrangement of bodies; and the concluding part of the work will be occupied with practical rules, derived from the facts and principles explained in the course of it, and applicable to the solving of various interesting problems in chemical analysis.
ELEMENTS
OF
EXPERI MENTAL CH E MISTRY.
PART I.
CHAPTER I.
OF A CHEMICAL LABORATORY AND APPARATUS,
A CHEMICAL laboratory, though extremely useful, and even essential, to all who embark extensively in the practice of chemistry, either as an art, or as a branch of liberal know¬ ledge, is by no means required for the performance of those simple experiments, which furnish the evidence of the funda¬ mental truths of the science. A room that is well lighted, easily ventilated, and destitute of any valuable furniture, is all that is absolutely necessary for the purpose. It is even ad- viseable, that the construction of a regular laboratory should be deferred, till the student has made some progress in the science ; for he will then be better qualified to accommodate its plan to his own peculiar views and convenience.
It is scarcely possible to offer the plan of a laboratory, which will be suitable to every person, and to all situations ; or to suggest any thing more than a few rules that should be gene¬ rally observed. Different apartments are required for the various classes of chemical operations. The principal one may be on the ground-floor; twenty-five feet long, fourteen or sixteen wide, and open to the roof, in which there should be contrivances for allowing the occasional escape of suffo¬ cating vapours. This will be destined chiefly for containing furnaces, both fixed and portable. It should be amply fur¬ nished with shelves and drawers, and with a large table in the
VOL. I. B
CHEMICAL APPARATUS*
CHAP. I.
centre, the best form of which is that of a double cross. Another apartment may be appropriated to the minuter ope¬ rations of chemistry; such as those of precipitation on a small scale, the processes that require merely the heat of a lamp, and experiments on the gases. In a third of smaller size, may be deposited accurate balances, and other instru¬ ments of considerable nicety, which would be injured by the acid fumes that are constantly spread through a laboratory.
The following are the principal instruments that are re¬ quired in chemical investigations; but it is impossible, with¬ out entering into very tedious details, to enumerate all the apparatus that should be in the possession of a practical chemist.
I. Furnaces. These may be formed either of solid brick¬ work, or of such materials as admit of their removal from place to place.
The directions generally laid down in elementary books of chemistry, for the construction of fixed furnaces, appear to me deficient in precision, and such as a workman would find it difficult to put in practice. I have, therefore, given plans and sections, in the last two plates, of the various kinds of furnaces ; and, in the Appendix, minute instructions will be found for erecting them *.
The furnaces of most general utility are, 1st, the Wind Furnace , in which an intense heat is capable of being excited for the fusion of metals, &c. In this furnace, the body sub¬ mitted to the action of heat, or the vessel containing it, is placed in contact with the burning fuel. Fig. 60 exhibits one of the most common construction. Fig. 61 is the section of a wind furnace ; the plan of which was obligingly communi¬ cated to me by Mr. Knight, of Foster-lane, London, to whom, also, I am indebted for that represented, fig. 62. The wind furnace of Mr. Chenevix is shown by fig. 74. 2dly, The Evaporating Furnace is formed of iron plates, joined together by rabbiting, and placed over horizontal re¬ turning flues of brick. Figs. 64 and 65, are two views of this
* See the Description of the 7th and 8th plates in the Appendix.
CHAP. I.
CHEMICAL APPARATUS.
$
furnace as recommended by Mr. Knight. When evaporation is performed by the naked fire, the vessel may be placed on the top of the furnace, fig. 60 or 61 ; and when effected through the intervention of a water bath, a shallow kettle of water, in which is placed the evaporating dish and its con¬ tents, may be set in the same situation. For the purposes of evaporating liquids, and drying precipitates on a small scale, at a temperature not exceeding 212° Faff, a convenient appa¬ ratus is represented by fig. 27. 3dly, The plan of a Rever¬ beratory furnace is exhibited by figs. 66, 67, and 68. 4thly, The Furnace far distilling by a Sand Heat is constructed by setting upon the top of the brick-work, fig. 60, the iron pot, fig. 71 ; a door being made in the side of the furnace for in- troducing fuel. Distillation by the naked fire is performed with the wind furnace, figs. 62, 63. 5thly, The Cupelling , or Enamelling Furnace , is shown by figs. 69, 70.
Portable furnaces, however, are amply sufficient for all the purposes of the chemical student, at the outset of his pursuit. The one which 1 prefer is that shown by figs. 58 and 59. It was originally contrived, I believe, by Mr. Schmeisser * ; and is made, with considerable improvements, and sold by Mr. Knight, and by other dealers, in chemical apparatus. Its size is so small, that it may be set on a table, and the smoke may be conveyed by an iron pipe, into the chimney of the apart¬ ment. In the furnace, as it is usually sold, the chimney, adapted for distillation with' a sand heat, passes directly through the sand-bath, the form of which is necessarily altered, from the common to a very inconvenient one. I have found it a great improvement to make the aperture for the chimney at k. This allows us to have a sand-bath of the usual shape, as shown by fig. 59 ; or even to place evapo¬ rating dishes, or a small boiler, on the top of the furnace. The aperture may be closed by a stopper, when we dispose the furnace as shown by fig. 28. Dr. Black’s furnace is gene¬ rally made of a larger size, and is adapted to operations on a more considerable scale. (See figs. 72 and 73.) Both these furnaces are constructed of thin iron plates, and are lined
* See his Mineralogy, Tab. iii. and iv.
i CHEMICAL APPARATUS. CHAP. L
with fire-clay. They will be minutely described in the refer¬ ences to the plates.
For the purpose of exciting a sudden heat, and of raising it to great intensity, nothing can be better adapted than a very simple, cheap, and ingenious furnace, contrived by Mr. Charles Aikin, fig. 55. It is formed out of pieces of black- lead melting pots, in a manner to be described in the Ap¬ pendix, and is supplied with air by a pair of double bellows, d. By a slight alteration, this furnace may occasionally be em¬ ployed for the operation of cupelling. (See fig. 57.)
II. For containing the materials, which are to be sub¬ mitted to the action of heat in a wind furnace, vessels called crucibles are employed. They are most commonly made of a mixture of fire clay and sand, occasionally with the ad¬ dition of plumbago, or black lead. The Hessian crucibles are best adapted for supporting an intense heat without melt¬ ing; but they are liable to crack when suddenly heated or cooled. The porcelain ones, made by Messrs. Wedgwood, are of much purer materials, but are still more apt to crack on sudden changes of temperature; and, when used, they should, therefore, be placed in a common crucible of larger size, the interval being filled with sand. The black-lead cru¬ cibles resist very sudden changes of temperature, and may be repeatedly used ; but they are destroyed when some saline sub¬ stances (such as nitre) are melted in them, and are consumed by a current of air. For certain purposes, crucibles are formed of pure silver, or platina. Their form varies consi¬ derably, as will appear from inspecting plate vi. figs. 49, 50, 51, and 54. It is necessary, in all cases, to raise them from the bars of the grate, by a stand, fig. 53, a or t. For the purpose of submitting substances to the continued action of a red heat, and with a considerable surface exposed to the air, the hollow arched vessel, with a flat bottom, fig. 52, termed a muffle, is commonly used. In fig. 69, d, e , the muffle is shown, placed in a furnace for use.
III. Evaporating vessels should ahvays be of a flat shape, so as to expose them extensively to the action of heat. (See
CHAP. I.
CHEMICAL APPARATUS
s
a section of one, fig. 12.) They are formed of glass, of earthen ware, and of various metals. Those of glass are with difficulty made sufficiently thin, and are often broken by change of temperature ; but they have a great advantage in the smoothness of their surface, and in resisting the action of most acid and corrosive substances. Evaporating vessels of porcelain, or Wedgwood’s ware, are next in utility, are less costly, and less liable to be cracked. They are made both of glazed and unglazed ware. For ordinary purposes the former are to be preferred; but the unglazed should be employed when great accuracy is required, since the glazing is acted on by several chemical substances. Evaporating vessels of glass, or porcelain, are generally bedded, up to their edge, in sand (see fig. 65) ; but those of various metals are placed immediately over the naked fire. When the glass or porcelain vessel is very thin, and of small size, as a watch glass for ex¬ ample, it may be held by means of a small prong, represented under fig. 12; or it may be safely placed on the ring of the brass stand, plate i. fig. 13, and the flame of an Argand’s lamp, cautiously regulated, may be applied beneath it. A lamp thus supported, so as to be raised or lowered, at plea¬ sure, on an upright pillar, to which rings, of various diame¬ ters, are adapted, will be found extremely useful ; and, when a strong heat is required, it is adviseable to employ a lamp, furnished with double concentric wicks. A lamp for burn¬ ing spirit of wine will, also, be found very convenient, espe¬ cially if provided (as they now generally are) with a glass cap to cover the wick when not in use, which, being fitted by grinding, prevents the waste of the spirit by eva¬ poration.
IV. In the process of evaporation, the vapour for the most part is allowed to escape; but of certain chemical processes, the collection of the volatile portion is the principal object. This process is termed distillation. It is performed in ves- vels of various forms and materials. The common still is so generally known, that a representation of it in the plates was deemed unnecessary It consists of a vessel, generally of
* See Aikin’s Chem. Diet. pi. ii. fig. 31.
6
CHEMICAL APPARATUS.
CHAP. I.
copper, shaped like a tea-kettle, but without its spout and handle. Into the opening of this vessel, instead of a common lid, a hollow moveable head is affixed, which ends in a nar¬ row, open pipe. This pipe is received into another tube of lead, which is twisted spirally, and fixed in a wooden tub, so that it may be surrounded by cold water. (Fig. 40, del.) When the apparatus is to be used, the liquid intended to be distilled is poured into the body of the still, and the head is fixed in its place, the pipe, which terminates it, being received into the leaden worm. The liquid is raised into vapour, which passes into the worm, is there condensed by the surrounding cold water, and flows out at the lower extremity.
The common still, however, can only be employed for vola¬ tilizing substances that do not act on copper, or other metals, and is, therefore, limited to very few operations. The vessel, fig. 2, is of glass, or earthen ware, and is also intended for distillation. It is termed an alembic , and consists of two parts; the body a for containing the materials, and the head b by which the vapour is condensed ; the pipe c conveying it to a receiver. Vessels, termed retorts , however, are more generally used. Fig. 1, a shows the common form, and fig. 13, a re¬ presents a stoppered, or tubulated retort. Retorts are made of glass, of earthen ware, or of metal. When a liquid is to be added at distant intervals during the process, the best con¬ trivance is that shown fig. 26, a, consisting of a bent tube, with a funnel at the upper end. When the whole is intro¬ duced at first, it is done either through the tubulure, or, if into a plain retort, through the funnel, fig. 10.
To the retort, a receiver is a necessary appendage; and this may either be plain, fig. 1, b9 or tubulated, as shown by the dotted lines at c. To some receivers a pipe is added (fig. 13, b), which may enter partly into a bottle beneath. This vessel, which is principally useful for enabling us to remove the dis¬ tilled liquid, at different periods of the process, is termed a quilled receiver. For some purposes, it is expedient to have the quilled part accurately ground to the neck of the bottle, c, which would then be furnished with a tubulure, or second neck, having a ground stopper, and should be provided, also, with a bent tube, to be occasionally applied, for conveying away any gases that may be produced. The condensation of
CHAP. I.
CHEMICAL APPARATUS.
1
the vapour is much facilitated, by lengthening the neck of the retort with an adopter (fig. 11), the wider end of which slips over the retort neck, while its narrow extremity is admitted into the mouth of the receiver. (See fig. 63.)
Heat may be applied to the retort in several modes. When the vessel is of earthen ware, and when the distilled substance requires a strong heat to raise it into vapour, the naked fire is applied, as shown fig. 63, Glass retorts are generally placed in heated sand (fig. 59); and, when of a small size, the flame of an Argand’s lamp, cautiously regulated, may be conve¬ niently used (fig. 13).
In several instances, the substance raised by distillation is partly a condensable liquid, and partly a gas, which is not condensed till it is brought into contact with water. To effect this double purpose, a series of receivers, termed JVoulfe's Apparatus , is employed. The first receiver (5, fig, 30) has a right-angled glass tube, open at both ends, fixed into its tu- bulure ; and the other extremity of the tube is made to ter- minate beneath the surface of distilled water, contained, as high as the horizontal dotted line, in the three-necked bottle c. From another neck of this bottle, a second pipe proceeds, which ends, like the first, under water, contained in a second bottle d . To the central neck a straight tube, open at both ends, is fixed, so that its lower end may be a little beneath the surface of the liquid. Of these bottles any number may be employed that is thought necessary.
The materials being introduced into the retort, the arrange¬ ment completed, and the joints secured in the manner to be presently described, the distillation is begun. The condens¬ able vapour collects in a liquid form in the balloon b , while the evolved gas passes through the bent pipe, beneath the sur¬ face of the water in c, which continues to absorb it till satu¬ rated. When the water of the .first bottle can absorb no more, the gas passes, uncondensed, through the second right® angled tube, into the water of the second bottle, which, in its turn, becomes saturated. Any gas that may be produced, which is not absorbable by wrater, escapes through the bent tube e, and may be collected, if necessary.
Supposing the bottles to be destitute of the middle necks.
s
CHEMICAL APPARATUS.
CHAP. I.
and, consequently, without the perpendicular tubes, the pro¬ cess would be liable to be interrupted by an accident : for if, in consequence of a diminished temperature, an absorption or condensation of gas should take place, in the retort u, and, of course, in the balloon 6, it must necessarily ensue that the water of the bottles c and d would be forced, by the pressure of the atmosphere, into the balloon, and possibly into the retort; but, with the addition of the central tubes, a sufficient quantity of air rushes through them to supply any .accidental vacuum. This inconvenience, however, is still more con¬ veniently obviated by Welther’s tube of safety (fig. 31, b\ which supersedes the expediency of three-necked bottles. The apparatus being adjusted, as shown by the figure, a small quantity of water is poured into the funnel, so as to about half fill the ball b. When any absorption happens, the fluid rises in the ball, till none remains in the tube, when a quan¬ tity of air immediately rushes in. On the other hand, no gas can escape, because any pressure from within is instantly fol¬ lowed by the formation of a high column of liquid in the per¬ pendicular part, which resists the egress of gas. This inge¬ nious invention I can recommend, from ample experience of its utility*
Very useful alterations in the construction of Woulfe’s ap¬ paratus have been contrived also by Mr. Pepys and Mr. Knight. That of the former is shown (fig. 32), where the balloon b is surmounted by a vessel accurately ground to it, and furnished with a glass valve, resembling that affixed to Nooth’s apparatus. This valve allows gas to pass freely into the vessel c, but prevents the water which it contains from falling into the balloon. Mr. Knight’s improvement is de¬ scribed, and represented in a plate, in the Philosophical Magazine, vol. xxf.
* Another modification of this apparatus, by Dr. Murray, is represented in Nich. Journ. 8vo. vol. iih or in Murray’s System of Chemistry, vol. i. pi. v. fig. 40. Fig. 41 of the same plate exhibits a cheap and simple form of this apparatus, contrived by the late Dr. Hamilton, and depicted originally in his translation of Berthollet on Dyeing. Mr. Burkitt’s im¬ provement of this apparatus may be seen in Nicholson’s Journal, 4to, vol. v. 349.
CHAP. T.
CHEMICAL APPARATUS.
9
When a volatile substance is submitted to distillation, it is necessary to prevent the escape of the vapour through the junctures of the vessels; and this is accomplished by the ap¬ plication of lutes. The most simple method of confining the vapour, it is obvious, would be to connect the places of junc¬ ture accurately together by grinding; and accordingly the neck of the retort is sometimes ground to the mouth of the receiver. This, however, adds too much to the expense of apparatus to be generally practised.
When the distilled liquor has no corrosive property (such as waiter, alcohol, ether, &c.), slips of moistened bladder, or of paper, or linen, spread with flour paste, white of egg, or mucilage of gum arabic, sufficiently answer the purpose. The substance which remains, after expressing the oil from bitter almonds, and which is sold under the name of almond-meal, or powder, forms a useful lute, when mixed, to the consist¬ ency of glaziers’ putty, with water or mucilage. For confining the vapour of acid, or highly corrosive substances, the fat lute is well adapted, it is is formed by beating perfectly dry and finely sifted tobacco pipe-clay, with painters’ drying oil, to such a consistence that it may be moulded by the hand. The same clay, beat up with as much sand as it will bear, without losing its tenacity, with the addition of cut towT, or of horse- dung, and a proper quantity of water, furnishes a good lute, which has the advantage of resisting a considerable heat, and is applicable in cases where the fat lute would be melted or destroyed. Various other lutes are recommended by chemical writers ; but the few that have been enumerated I find to be amply sufficient for every purpose.
On some occasions, it is necessary to protect the retort from too sudden changes of temperature, by a proper coating;. For glass retorts, a mixture of moist common clay, or loam, with sand, and cut shreds of tow or flax, may be employed. If the distillation be performed by a sand heat, the coating needs not to be applied higher than that part of the retort which is bedded in sand ; but if the process be performed in a wind furnace (fig. 63), the whole body of the retort, and that part of the neck also which is exposed to heat, must be carefully coated. To this kind of distillation, however, earthen retorts
2
10
CHEMICAL APPARATUS,
CHAP. I,
are better adapted ; and they may be covered with a compo¬ sition originally recommended by Mr. Willis. Two ounces of borax are to be dissolved in a pint of boiling water, and a sufficient quantity of slaked lime added, to give it the thick¬ ness of cream. This is to be applied by a painter’s brush, and allowed to dry. Over this a thin paste is afterwards to be applied, formed of slaked lime and common linseed-oil, well mixed and perfectly plastic. In a day or two, the coating will be sufficiently dry to allow the use of the retort.
For joining together the parts of iron vessels, used in distil¬ lation, a mixture of the finest China clay, with solution of borax, is well adapted. In all cases, the different parts of any apparatus made of iron should be accurately fitted by boring and grinding, and the above lute is to be applied to the part which is received into an aperture. This wall generally be sufficient without any exterior luting : otherwise the lute of clay, sand, and flax, already described, may be used.
In every instance, where a lute or coating is applied, it is adviseable to allow it to dry before the distillation is begun ; and even the fat lute, by exposure to the air during one or two days after its application, is much improved in its quality. The clay and sand lute is perfectly useless, except it be pre¬ viously quite dry. In applying a lute, the part immediately over the juncture should swell outwards, and its diameter should be gradually diminished on each side. (See fig. 13, where the luting is shown, applied to the joining of the retort and receiver.)
Beside the apparatus already described, a variety of vessels and instruments are necessary, having little resemblance to each other, in the purposes to which they are adapted. Glass vessels are required for effecting solution , which often re¬ quires the application of heat, and sometimes for a consider¬ able duration. In the latter case, it is termed digestion, and the vessel, fig. 4, called a matrass , is the most proper for per¬ forming it. When solution is required to be quickly effected, the bottle, fig 5, with a rounded bottom, may be used ; or a common Florence oil flask serves the same purpose extremely well, and bears, without cracking, sudden changes of tempe-
CHAP. I®
CHEMICAL APPARATUS.
II
rature. For precipitations , and separating liquids from preci¬ pitates, the decanting-jar (fig. 14), will be found useful; or, if preferred, it may be shaped as in fig. 26,/. Liquids, of dif¬ ferent specific gravities, are separated by the vessel, fig. 3 ; the heavier fluid being drawn off through the cock b, and air being admitted by the removal of the stopper a, to supply its place. Glass rods, of various lengths, and spoons of the same material, or of porcelain, are useful for stirring acid and cor¬ rosive liquids ; and a stock of cylindrical tubes, of various sizes, is required for occasional purposes. It is necessary also to be provided with a series of glass measures, graduated into drachms, ounces, and pints. The small tube, fig. 15, called a dropping tube , which is open at each end and blown in the middle into a ball, will be found useful in directing a fine stream of water upon the edges of a filtre, or any small ob¬ ject. The same purpose may, also, be very conveniently effected by fixing a piece of glass tube of small bore, two or three inches long, and bent at one end to an obtuse angle, into a hole bored in a cork, which may be used as the stopple of an eight ounce vial filled with water, fig. 25, a . On in¬ verting the vial, and grasping the bottom part of it, the warmth of the hand expels either a few drops or a small stream of water, which may be directed upon any minute object. When the flow ceases, it may be renewed, if required, by setting the bottle, for a moment, with its mouth upwards (which admits a fresh supply of cool air), and then proceeding as before.
For the drying of precipitates, and other substances, by a heat not exceeding 212°, a very useful apparatus is sold in London. It is represented, supported by the ring of a lamp- stand, by fig. 27. The vessel a is of sheet-iron or copper japanned and hard-soldered ; c is a conical vessel of very thin glass, having a rim, which prevents it, when in its place, from entirely slipping into a; and cl is a moveable ring, which keeps the vessel c in its place. When the apparatus is in use, water is poured into a about as high as the dotted line ; the vessel c, containing the substance to be dried, is immersed in the water, and secured by the ring d ; and the whole apparatus set over an Argand’s lamp. The steam escapes by means of the chim¬ ney 5, through which a little hot water may be occasionally poured, to supply the waste by evaporation. By changing
12
CHEMICAL APPARATUS.
CHAP. I.
the shape of c to the segment of a sphere, still retaining the rim, I have found it a most convenient vessel for evaporating
fluids.
Accurate beams and scales, of various sizes, with corres¬ ponding weights, some of which are capable of weighing seve¬ ral pounds, while the smaller size ascertains a minute fraction of a grain, are essential instruments in the chemical labora¬ tory. So also are mortars of different materials, such as of glass, porcelain, agate, and metal. Wooden stands, of various kinds, for supporting receivers, should be provided * * * §. For purposes of this sort, and for occasionally raising to a proper height any article of apparatus, a series of blocks, made of well seasoned wood, eight inches (or any other number) square, and respectively eight, four, two, one, and half an inch in thickness, will be found extremely useful ; since, by combining them in different ways, thirty-one different heights may be obtained.
The blow-pipe is an instrument of much utility in chemical researches. A small one, invented by Mr. Pepys, with a flat cylindrical box for condensing the vapour of the breath, and for containing caps, to be occasionally applied with apertures of various sizes, is perhaps the most commodious form t. One of a much smaller size, for carrying in the pocket, has been contrived by Dr. Wollaston J. A blow-pipe, which is sup¬ plied with air from a pair of double bellows, worked by the foot §, may be applied to purposes that require both hands to be left at liberty, and will be found useful in blowing glass, and in bending tubes. The latter purpose, howrever, may be accomplished by holding them over an Argand’s lamp with double wicks. Occasionally, when an intense heat is required, the flame of the blow-pipe, instead of being supported by the mouth, may be kept up by a stream of oxygen gas, expelled from a bladder or from a gas-holder |] . The blow-pipe invented by Mr. Brooke consists of a small square box of
* See Aikin’s Cliem. Diet. pi. iv. fig. 59, e.
f See Aikin’s Chem. Diet. pi. vii. fig. 71, 72, 73.
J It is described in Nich. Journ. xv. 284.
§ Phil. Mag. xliii. 280.
jj See a representation of the apparatus for this purpose, in the Chemical Conversations, pi. ix.
CHAP. I.
CHEMICAL APPARATUS.
15
copper or iron, into which air is forced by a condensing syringe, and from which it is suffered to rush, through a tube of very small aperture, regulated by a stop-cock, against the flame of a lamp or candle #. By means of a screw added to the syringe, the receiver may be filled with oxygen gas, or, as will be described in chap. v. sect. 5, with a mixture of hy¬ drogen and oxygen gases. Blow-pipes on this construction may be had of Mr. Newman, and of most of the other makers of philosophical instruments.
In the course of this work, various other articles of appa¬ ratus will be enumerated, in detailing the purposes to which they are adapted, and the principles on which they are con¬ structed. It must be remembered, however, that it is no part of my object to describe every ingenious and complicated in¬ vention, which has been employed in the investigation of che¬ mical science: but merely to assist the student in attaining apparatus for general and ordinary purposes. For such pur¬ poses, and even for the prosecution of new and important in¬ quiries, very simple means are sufficient; and some of the most interesting chemical facts may be exhibited and even ascertained, with the aid merely of Florence flasks, of com¬ mon vials, and of wine glasses. In converting these to the purposes of apparatus, a considerable saving of expense will accrue to the experimentalist; and he will avoid the encum¬ brance of various instruments, the value of which consists in show, rather than in real utility.
In the selection of experiments, I shall generally choose such as may be undertaken by persons not possessed of an extensive chemical apparatus. On some occasions, however, it may be necessary, in order to complete the series, that others should be included, requiring, for their performance, instruments of considerable nicety. The same experiment may, perhaps, in a few instances, be repeatedly introduced in illustration of different principles; but this repetition will be avoided as much as possible. Each experiment will be pre¬ ceded by a brief enunciation of the general truth which it is intended to illustrate.
* Thomson’s Annals, vii. 367 ; or, Journal of Science and the Arts, i.
u
CHAPTER II.
OF CHEMICAL AFFINITY.
All bodies, composing the material system of the universe, have a mutual tendency to approach each other, whatsoever may be the distances at which they are placed. The opera¬ tion of this force extends to the remotest parts of the planetary system, and is one of the causes that preserve the regularity of their orbits. The smaller bodies, also, that are under our more immediate observation, are influenced by the same power, and fall to the Earth’s surface, when not prevented by the interference of other forces. From these facts, the existence of a property has been inferred, which has been called attraction , or more specifically, the attraction of gravita¬ tion. Its nature is entirely unknown to us ; but some of its laws have been investigated, and successfully applied to the explanation of phenomena. Of these, the most important are, that the force of gravity acts on bodies directly in proportion to the quantity of matter in each ; and that it decreases in the reciprocal proportion of the squares of the distances.
From viewing bodies in the aggregate, we may next pro¬ ceed to contemplate them as composed of minute particles. Of the nature of these particles, we have no satisfactory evi¬ dence. It is probable that they consist of solids, which are incapable of mechanical division, but are still possessed of the dimensions of length, breadth, and thickness. In simple bodies, the particles must be all of the same nature, or homo¬ geneous. In compound bodies, we are to understand, by the term, particles , the smallest parts into which bodies can be resolved wfithout decomposition. The word atom has of late been revived, to denote both these kinds of particles ; and we may, therefore, speak with propriety of simple atoms and of compound atoms . When two atoms of different kinds unite to form a third or compound atom, we may term the two first component atoms ; and if these have not been decomposed, they may be called elementary or primary atoms.
CHAP. II. CHEMICAL AFFINITY, &C. IS
The atoms or particles of bodies are also influenced by the force of attraction, but not unless when placed in apparent contact. Hence a distinction has been made between gravita¬ tion, and that kind of attraction which is effective only at insensible distances. The latter has been called contiguous attraction or affinity ; and it has been distinguished, as it is exerted between particles of matter, of the same kind, or be¬ tween particles of a different kind.
By the affinity of aggregation, the cohesive affinity , or, more simply cohesion , is to be understood that force or power, by which particles or atoms of matter of the same kind attract each other, the only effect of this affinity being an aggregate or mass. Thus a lump of copper may be considered as composed of an infinite number of minute particles or integrant parts, each of which has precisely the same properties, as those that belong to the whole mass. These are united by the force of cohesion. But if the copper be combined with another metal (such as zinc), we obtain a compound (brass), the constituent parts of which, copper and zinc, are combined by the power of chemical affinity. In simple bodies, therefore, cohesion is the only force exerted between their particles. But in com¬ pound bodies, we may distinguish the force, with which the component atoms are united, from that which the compound atoms exert towards each other: the former being united bv chemical affinity, and the latter by the cohesive attraction.
SECTION I.
Of Cohesion , Solution , and Crystallization .
The cohesive affinity is a property, which is common to a great variety of bodies. It is most strongly exerted in solids ; and in these it is proportionate to the mechanical force re¬ quired for effecting their disunion. In liquids, it acts with considerably less energy ; and in aeriform bodies we have no evidence that it exists at all ; for their particles, as will after¬ wards be shown, are mutually repulsive, and, if hot held to-
6
16
CHEMICAL AFFINITY, &C.
CHAP. II.
gether by pressure, would probably separate to immeasurable distances. Its force is not only different among different bodies, but in various states of the same body. Thus in the cohesion of certain metals (steel for instance), important changes are produced by the rate of cooling, by hammering, and by other mechanical operations. W ater, also, in a solid state, has considerable cohesion, which is much diminished when it becomes liquid, and is entirely destroyed when it is changed into vapour.
The most important view, in which the chemist has to con¬ sider cohesion, is that of a force either counteracting; or modi- fying chemical affinity; for the more strongly the particles of any body are united by this power, the less are they disposed to enter into combination with other bodies. In many cases, a very powerful affinity existing between two substances may be rendered wholly inefficient, by the strong cohesion of one or both of them. Hence it has been received as an axiom, that the affinity of composition is inversely proportionate to the cohesive affinity. To the language, however, in which this axiom is expressed, it has been justly objected, that it implies an accuracy of proportion between the forces of cohesion and of chemical affinity, which cannot be proved to exist; since all that can truly be affirmed is, in general terms, that the affinity of composition is less effective, as the attraction of cohesion is stronger.
The cohesion of bodies may be overcome, 1st, by me¬ chanical operations, as by rasping, grinding, pulverising, and other modes of division, which are generally employed as pre¬ liminary steps to chemical processes. In some instances, even a minuter division of bodies is necessary, than can be accom¬ plished by mechanical means; and recourse is then had to precipitation. Silica, for example, in the state of rock crystal, may be boiled for a long time in liquid potash, without any appearance of chemical action. It may even be bruised to the finest powder, without being rendered sensibly soluble. But when first precipitated from a state of chemical solution, it is readily dissolved by that menstruum.
2dly. Cohesion may be counteracted by heat, applied so as to melt one or both of the bodies, if fusible; or to raise them
*
SECT. I.
CHEMICAL AFFINITY, &C.
17
into vapour, if volatile. Lead and sulphur contract no union, till one or both of them is melted by heat. Arsenic and sul¬ phur are united most effectually, by bringing them into con¬ tact, when both are in a state of vapour.
3dly. Cohesion may be counteracted by solution ; and this is so general a condition of chemical: union, that it was formerly received as an axiom, that bodies do not act on each other , unless one or both are in a state of solution ; a principle, to which the progress of chemical science has since discovered many exceptions.
The term solution is applied to a very extensive class of phe¬ nomena. When a solid disappears in a liquid, or when a solid or liquid is taken up by an aeriform body, if the compound exhibit perfect transparency, we have, in each instance, an example of solution. The expression is applied, both to the act of combination, and to the result of the process. When common salt, such as is used in cookery, is agitated with water, it disappears ; in other words, its solution takes place ; and we also term the liquid which is obtained, a solution of salt in water. This is one of the simplest cases that can be adduced, of the efficiency of chemical affinity ; for solution is always the result of an affinity between the fluid and the solid which is acted upon, sufficient in force to overcome the co¬ hesion of the solid. This affinity continues to act, until, at length, a certain point is attained, where the affinity of the solid and fluid for each other is overbalanced by the cohesion of the solid, and the solution cannot be carried farther. This point is called saturation , and the fluid obtained is termed a saturated solution .
With respect to common salt, water acquires no increase of its solvent power by the application of heat. But there are various salts with which water may be saturated at the common temperature of the atmosphere, and will yet be ' capable of dissolving a still farther quantity by an increase of its temperature. When a solution, thus charged with an ad¬ ditional quantity of salt, is allowed to cool, the second portion of salt is deposited in a form resembling its original one.
To recover a salt from its solution, if its solubility does not vary with the temperature of the solvent, as in the instance
VOL. i. c
CHEMICAL AFFINITY, &C.
CHAP. II,
II
of common salt, it is necessary to expel a portion of the fluid by heat. This constitutes the process of evaporation. II the evaporation be carried on very slowly, so that the particles oi the solid may approach each other in the way best adapted to them, we obtain solid figures, of a regular shape, called crys¬ tals . The crystallization of a solid may also take place from that state of fluidity which is produced by heat. Thus several of the metals crystallize on cooling from a melted state ; and some volatile bodies, as arsenic, assume, when condensed from the state of vapour, the shape of regular crystals.
In the act of separating from the water in which they were dissolved, the crystals of almost all salts carry with them a quantity of water, which is essential to the regularity of their form, and cannot be expelled without reducing them to shape¬ less masses. It is termed their water of crystallization. Its proportion varies in different salts; in some it is extremely small ; in others it constitutes the principal part of the salt, and is even so abundant, as to liquefy them on the applica¬ tion of heat, producing what is called the watery fusion. The water of crystallization is retained also in different salts with very different degrees of force. Some crystals, which lose their watery ingredient by mere exposure to the atmosphere, are said to effloresce. Others, on the contrary, not only hold their water of crystallization very strongly, but even attract more; and, on exposure to the atmosphere, become liquid, or deliquiate. The property itself is called deliquescence .
When two salts are contained in the same solution, which vary, in their degree of solubility, and which have no remark¬ able attraction for each other, they may be obtained separate. For by carefully reducing the quantity of the solvent by eva¬ poration, the salt whose particles have the greatest cohesion, will crystallize first. If both salts are more soluble in hot than in cold water, the crystals will not appear till the liquid cools. But if one of them, like common salt, is equally solu¬ ble in hot and in cold water, crystals will appear, even during the act of evaporation. In this way we may completely sepa¬ rate nitre from common salt, the crystals of the latter being formed during evaporation ; while those of nitre do not appear till some time after the fluid has cooled.
SECT* I. ' CHEMICAL AFFINITY, &C. 19
Salts, which are thus deposited in regular shapes, generally adhere to the surface of the vessel containing the solution, or to any substance, such as pieces of thread or of wood, intro¬ duced for the purpose of collecting them. But a still more effectual way of inducing crystallization is to immerse, in the solution, a crystal of the same kind with that which we expect to be formed. The crystal, thus exposed, receives successive additions to its several surfaces, and preserves its form, with a considerable addition to its magnitude. This curious fact was originally noticed by Le Blanc, who has founded on it a method of obtaining large and perfect crystals.
In some instances, the affinity of a salt for its solvent is so powerful, that it will not separate from it in the form of crys¬ tals; but will yet crystallize from another fluid, which is capable of dissolving it, and for which it has a weaker affinity. Pot-ash, for instance, cannot be made to crystallize from its watery solution, but will yet separate, in a regular form, from its solution in alcohol.
Every solid, that is susceptible of crystallization, has a tendency to assume a peculiar shape. Thus common salt, when most perfectly crystallized, forms regular cubes ; nitre has the shape of a six-sided prism ; and alum that of an oc¬ tahedron. It has, indeed, been alleged, as an objection to the modern theory of crystallization, that minerals, differing essen¬ tially in their composition, have precisely the same primitive form. For example, the primitive form of carbonate of lime, and of the compound carbonate of lime and magnesia, is, in both, a regular rhomboid, so nearly resembling each other, as to have been supposed to be precisely the same. In this case, however, Dr. Wollaston has shown, that though the figures are similar, yet their angles, on admeasurement by a nice instrument, differ very appreciably*. But other instances have been since brought forward by M. Beudant, in which artifical salts, composed of dissimilar ingredients, have the same crystalline form ; and Dr. Wollaston has satisfied himself of the accuracy of M. Beudant’s remark, that the mixed sul¬ phates of copper and iron, of zinc and iron, and of copper
* Phil. Trans. 1812. c 2
25
CHEMICAL AFFINITY, &C.
CHAP. II.
zinc and iron, assume forms, in which no difference has yet been discovered from that of simple sulphate of iron alone*. He apprehends, indeed, that on minute investigation, some difference will be found, either in the angles or linear measures of those different salts ; but till this has been established, the facts, as they stand, must be acknowledged to be exceptions to the principle, that identity of crystalline form is necessarily connected with identity of chemical composition. In the instances which have been given, the perfect transparency of the crys¬ tals forbids our considering them as an intermixture of foreign matter grouped together by sulphate of iron ; and this expla¬ nation is, also, irreconcileable with the fact, discovered by Dr. Wollaston, that a mixed solution of sulphates of zinc and copper, in certain proportions, affords crystals which, though containing no iron, still agree so nearly in form with those of sulphate of iron, that he could not undertake to point out any difference between them.
It has been long known that the same solid admits of great varieties of crystalline figure, without any variation of its che¬ mical composition. Calcareous spar, for example, appears in six-sided prisms, in three or six-sided pyramids, anti in many other shapes. These varieties are occasioned by accidental cir¬ cumstances, which modify the operation of the force of cohe¬ sion. The diversities of shape are, on first view, extremely numerous ; and yet, upon a careful examination and compari¬ son, they are found to be reducible to a small number of simple figures, which, for each individual species, is always the same.
The attempt to trace all the observed forms of crystals to a few simple or primary ones, seems to have originated with Bergman f. In the instance of calcareous spar, this distin¬ guished chemist demonstrated that its numerous modifications may possibly result from one simple figure, the rhomb, by the accumulation of which, in various ways, crystals of the most opposite forms may be generated. This theory he ex¬ tended to crystals of every kind; and he accounted for the differences of their external figures, by varieties of their me¬ chanical elements or minute molecules.
* Thomson's Annals, xi. 262, 283.
f Bergman's Essays, ii.
SECT. I.
CHEMICAL AFFINITY, &C.
n
About the same period with Bergman, or immediately afterwards, M. Rome de l’Isle pursued still farther the theory of the structure of crystals. He reduced the study of crystal- lography to principles more exact, and more consistent with observation. He classed together, as much as he was able, crystals of the same nature. From among the different forms belonging to the same species, he selected, for the primitive form, one which appeared to him to be the most proper, on account of its simplicity. Supposing this to be truncated in different manners, he deduced the other forms, and established a certain gradation, or series of passages, from the primitive form to complicated figures, which on first view would scarcely appear to have any connexion with it. To the descriptions and figures of the primitive forms, he added the mechanical measurement of the principal angles, and showed that these angles are constantly the same in each variety. It must be acknowledged, however, that the primitive forms, assumed by this philosopher, were entirely imaginary, and not the result of any experimental analysis. His method was to frame an hypothesis ; and then to examine its coincidence with actual appearances. On his principles any form might have been the primitive one, and any other have been deduced from it.
It was reserved for the sagacity of the Abbe Haiiy to unfold the true theory of the structure of crystals, and to support it both by experimental and mathematical evidence. By the mechanical division of a complicated crystal, he first obtains the simple form, and afterwards constructs, by the varied ac¬ cumulation of the primitive figure, according to mathematical synthesis, all the observed varieties of that species.
Every crystal may be divided by means of proper instru¬ ments ; and, if split in certain directions, presents plane and smooth surfaces. If split in other directions, the fracture is rugged, is the mere effect of violence, and is not guided by the natural joining of the crystal. This fact had been long known to jewellers and lapidaries; and an accidental obser¬ vation of it proved, to the Abbe Haiiy, the key of the whole theory of crystallization. By the skilful division of a six- sided prism of calcareous spar, he reduced it to a rhomb, pre¬ cisely resembling that which is known under the name of Ice-
22
CHEMICAL AFFINITY, &C.
CHAP. II.
land crystal. Other forms of calcareous spar were subjected to the same operation ; and, however different at the outset, finally agreed in yielding, as the last product, a rhomboidal solid. It was discovered also by Haiiy, that if we take a crystal of another kind (the cubic fluor spar for instance), the nucleus, obtained by its mechanical division, will have a dif¬ ferent figure, viz. an octahedron. Other crystallized bodies produce still different forms ; which are not, however, very numerous. Those which have hitherto been discovered, are reducible to six ; the parallelopipedon, which includes the cube, the rhomb, and all the solids which are terminated by six faces, parallel two and two ; the tetrahedron ; the octahe¬ dron ; the regular hexaliedral prism ; the dodecahedron with equal and similar rhomboidal planes ; and the dodecahedron with triangular planes.
The solid of the primitive form or, nucleus of a crystal ob¬ tained by mechanical division, may be subdivided in a direc¬ tion parallel to its different faces. All the sections thus pro¬ duced being similar, the resulting solids are precisely similar in shape to the nucleus, and differ from it only in size, which continues to decrease as the division is carried farther. To this division, however, there must be a limit, beyond which we should come to particles so small, that they could no lon¬ ger be divided. At this term, therefore, wre must stop : and to these last particles, the result of an analysis of the primitive nucleus, and similar to it in shape, Haiiy has given the name of the integrant molecule. If the division of the nucleus can be carried on in other directions than parallel to its faces, the integral molecule may then have a figure different from that of the nucleus. The forms, however, of the integrant mole¬ cule, which have hitherto been discovered, are only three ; the tetrahedron, the simplest of pyramids ; the triangular prism, the simplest of prisms ; and the parallelopipedon, including the cube and rhomboid, the simplest of solids which have their faces parallel two and two. With respect to octahedral crystals, there is a difficulty, whether the octahedron, or tetrahedron, is to be adopted as the primitive form ; and, whichsoever be chosen, since neither of them can fill space without leaving vacuities, it is not easy to conceive any ar-*
Sfi€T f.
CHEMICAL AFFINITY, &C.
2S
rangement, by which the particles will remain at rest. To obviate this difficulty, Dr. Wollaston has suggested that, in such instances, the elementary particles may be perfect spheres ; and by the due application of spheres to each other? he has shown, that a variety of crystalline forms may be pro¬ duced*; viz. the octohedron, the tetrahedron, and the acute rhomboid. If other particles, having the same relative ar¬ rangement, be supposed to have the shape of oblate spheroids, the regular rhomboid will be the resulting figure ; 'and if the spheroids be oblong instead of oblate, they will generate prisms of three or six sides. The cube, also, Dr. Wollaston has shown, may be explained by the aggregation of spheroidical particles.
A method of developing the structure of crystals, by a new process, which appears greatly superior to that of mechanical divisions, has been lately described by Mr. Daniel f. It con¬ sists in exposing any moderately soluble salt to the slow and regulated action of a solvent. A shapeless mass of alum, for instance, weighing about 1500 grains, being immersed in 15 ounce measures of water, and set by, in a quiet place, for a period of three or four weeks, will be found to have been more dissolved toward the upper than the lower part, and to have assumed a pyramidal form. On further exa¬ mination, the lower end of the mass will present the form of octahedrons and sections of octahedrons, in high relief and of various dimensions. They will be most distinct at its lower extremity, becoming less so as they ascend. This new process of dissection admits of exclusive application. Borax, in the course of six weeks, exhibits eight sided prisms with various terminations; and other salts may be made to un¬ fold their external structure by the slow agency of water. Car¬ bonate of lime, carbonate of strontites, and carbonate of barytes, give also distinct results, when acted upon by weak acids; and even amorphous masses of those metals, which have a tendency to assume a crystalline form, such as bismuth, antimony, and nickel, when exposed to very dilute nitric acid, presented at the end of a few days distinct crystalline forms. The results of these experiments, when minutely traced and
* PhiL Trans. 1813, p. 51. + Jour, of Science and the Arts, i. 94.
24?
CHEMICAL AFFINITY, &C.
CHAP. II.
investigated, as has been ably done in Mr. Daniel’s Memoir, afford strong confirmation to the theory, that the spheroidical is the true form of the ultimate particles of crystallized bodies.
The primitive form, and that of the integral molecule hav¬ ing been experimentally determined by the dissection of a crystal, the next step is to discover the law, acording to which these molecules are arranged, in order to produce, by their accumulation around the primitive figure, the great variety of secondary forms. What is most important in the discoveries of Haiiy, and what constitutes in fact the essence of his theory, is the determination of these laws, and the precise measure¬ ment of their action. Fie has shown that all the parts of a secondary crystal, superadded to the primitive nucleus, con¬ sist of laminae, which decrease gradually by the subtraction of one or more layers of integrant molecules; so that theory is capable of determining the number of these ranges, and, by a necessary consequence, the exact form of the secondary crystal.
By the developement of these laws of decrement, Haiiy has shown how, from variations of the arrangement of the integrant molecules, a great variety of secondary figures may be produced. Their explanation, however, would involve a minuteness of detail, altogether unsuitable to the purpose of this work ; and I refer, therefore, for a very perspicuous state¬ ment of them, to the first and ninth volumes of the Philoso¬ phical Magazine.
SECTION II.
Of Chemical Affinity , and the General Phenomena of Chemical
Action .
Chemical affinity, like the cohesive attraction, is effective only at insensible distances ; but it is distinguished from the latter force, in being exerted between the particles or atoms of bodies of different kinds. The result of its action is not a mere aggregate, having the same properties as the separate parts, and differing only by its greater quantity or mass, but a new compound, in which the properties of the components have either entirely or partly disappeared, and in which new
SECT II.
CHEMICAL AFFINITY, &C.
25
qualities are also apparent. The combinations effected by chemical affinity are permanent, and are destroyed only by the interference of a more powerful force, either of the same or of a different kind.
As a general exemplification of chemical action, we may assume that which takes place between potash and sulphuric acid. In their separate state, each of these bodies is distin¬ guished by striking peculiarities of taste, and by other quali¬ ties. The alkali, on being added to blue vegetable infusions, changes their colour to green ; and the acid turns them red. But if wre add the one substance to the other, very cautiously and in small quantities, examining the effect of each addition, we shall at length attain a certain point, at which the liquid will possess neither acid nor alkaline qualities ; the taste will be converted into a bitter one ; and the mixture will produce no effect on blue vegetable colours. Here then, the qualities of the constituent parts, or at least some of their most im¬ portant ones, are destroyed by combination. When opposing properties thus disappear, the bodies combined have been said to saturate each other ; and the precise term, at which this takes place, has been called the point of saturation. It is adviseable, however, to restrict this expression to weaker com¬ binations, where there is no remarkable alteration of qualities, as in cases of solution ; and to apply to those results of more energetic affinities, which are attended with Joss of properties, the term neutralization .
At the same time that the properties of bodies disappear on combination, other new qualities, both sensible and chemical, are acquired ; and the affinities of the components for other substances become in some cases increased, in others dimi¬ nished in energy. Sulphur, for example, is destitute of taste, smell, or action on vegetable colours ; and oxygen gas is, in these respects, equally inefficient. But the compound of sul¬ phur and oxygen is intensely acid ; the minutest portion in¬ stantly reddens blue vegetable infusions; and the acid is dis¬ posed to enter into combination with a variety of bodies, for which its components evinced no affinity. Facts of this kind sufficiently refute the opinion of the older chemists, that the properties of compounds are intermediate between those of their
tQ CHEMICAL AFFINITY, &C. CHAP. II.
f
component parts; for, in instances like the foregoing, the compound has qualities, not a vestige of which can be traced to either of its elements.
It is not, however, in all cases, that the change of properties is so distinct and appreciable by the senses, as in the instances which have been just now described. In some examples of chemical union, the change is scarcely perceptible to the eye or taste, when the chemist is nevertheless certain that combi¬ nation must have taken place. This occurs chiefly in the mixture of saline solutions with each other, where a complete exchange of principles ensues, without any evident change of properties. Examples of this kind cannot, however, be un¬ derstood, till the subject of complex affinity has been first elu¬ cidated .
The existence of chemical affinity between any two bodies is inferred, therefore, from their entering into chemical com¬ bination ; and that this has happened, a change of properties may be considered as a sufficient proof, even though the change may not be very obvious, and may require accurate examination to be perceived at all.
The proof, which establishes the nature of chemical com¬ pounds, is of two kinds, synthesis and analysis . Synthesis consists in effecting the chemical union of two or more bodies ; and analysis in separating them from each other, and exhibit¬ ing them in a separate state. When we have a compound of two or more ingredients, which are themselves compounded also, the separation of the compounds from each other may be called the proximate analysis of the body ; and the farther separation of these compounds into their most simple prin¬ ciples, its ultimate analysis . Thus the proximate analysis of sulphate of potash consists in resolving it into potash and sul¬ phuric acid ; and its ultimate analysis is effected by decom¬ posing the potash into potassium and oxygen, and the sul¬ phuric acid into oxygen and sulphur.
When the analysis of any substance has been carried as far as possible, we arrive at its most simple principles, or elements , by which expression we are to understand, not a body that is incapable of further decomposition, but only one which has not yet been decomposed . The progress of chemical science.
SECT. IT.
CHEMICAL AFFINITY, &C.
27
for several centuries past, lias consisted in carrying still farther the analysis of bodies, and in proving those to be com¬ pounded, which had before been considered as elementary.
Beside the alteration of properties, which usually accom¬ panies chemical action, there are certain other phenomena, which are generally observed to attend it.
1st. In almost every instance of chemical union, the specific gravity of the compound is greater than might have been in¬ ferred from that of its components; and this is true both of weaker and more energetic combinations. When equal weights of water and sulphuric acid are made to combine, the specific gravity of the resulting liquid is not the mean, but considerably greater than the mean. The law extends also to solids. But though general, it is not universal ; for in a very few instances, chiefly of aeriform fluids, condensation does not attend chemical union. And in the combination of metals with each other the reverse even takes place, the com¬ pound being specifically lighter than might have been ex¬ pected, from the specific gravity of its elements, and their proportion to each other.
2dly. When bodies combine chemically, it may be received as a general fact, that their temperature changes. Equal weights of oil of vitriol and water, both at the temperature of 50° of Fald., are heated, by sudden mixture, to considerably above 212°. In other examples, a contrary effect takes place, and a diminished temperature, or, in other words, a produc¬ tion of cold, is observed. This is all that it is at present necessary to state on the subject, which will be more fully considered when we come to treat of caloric.
3dly. The forms of bodies are often materially changed by chemical combination. Two solids may, by their union, be¬ come fluid ; or two fluids may become solid. Solids are also often changed into aeriform fluids; and, in many instances, the union of two airs, or gases, is attended with their sudden conversion into the solid state. By long exposure of quick¬ silver to a moderate heat, we change it from a brilliant liquid into a reddish scaly solid ; and by heating this solid in a re¬ tort, we obtain an aeriform fluid, or gas, in considerable quantity, and recover the quicksilver in its original form.
6
28
CHEMICAL AFFINITY, &C.
CHAP. II.
4thly. Change of colour is a frequent, but not universal concomitant of chemical action. In some cases, brilliant colours are destroyed, as when oxymuriatic acid is made to act on solution of indigo. In other instances, two substances, which are nearly colourless, form, by their union, a com¬ pound distinguished by beauty of colour, as when liquid pot¬ ash is added to a very dilute syrup of violets. Certain colours appear also to belong essentially to chemical compounds, and to be characteristic of them. Thus 100 parts of quicksilver, and 4 of oxygen, invariably give a black compound; and the same quantity, with 8 parts of oxygen, a red compound.
SECTION III.
Of the Proportions in which bodies combine ; and of the Atomic
In the chemical combination of bodies with each other, a few leading circumstances deserve to be remarked.
1st. Some bodies unite in all proportions; for example, water and sulphuric acid, or water and alcohol.
2dly. Other bodies combine in all proportions, as far as a certain point, beyond which combination no longer takes place. Thus water will take up successive portions of com¬ mon salt, until at length it becomes incapable of dissolving any more. In cases of this sort, as well as in those included under the first head, combination is wreak and easily destroyed, and the qualities wdiich belonged to the components in their separate state continue to be apparent in the compound.
3dly. There are many examples in which bodies unite in one proportion only ; and in all such cases the proportion of the elements of a compound must be uniform for the species. Thus hydrogen and oxygen unite in no other proportions, than those constituting water, which, by weight, are very nearly 11 a of the former to 88A of the latter, or 1 to 7 a* In. cases of this sort, the combination is generally energetic ; and the characteristic qualities of the components are no longer observable in the compound.
SECT. III. CHEMICAL AFFINITY, &C. 29
4thly. Other bodies unite in several proportions : but these proportions are definite, and, in the intermediate ones, no combination ensues. Thus 100 parts by weight of charcoal combine with 1324- of oxygen, or with 2 65, but with no in¬ termediate quantity; 100 parts of manganese combine with 14 of oxygen, or with 28, or with 42, or with 56, and with those proportions only.
Now it is remarkable, that when one body enters into com¬ bination with another, in several different proportions, the numbers indicating the greater proportions are exact simple multiples of that denoting the smallest proportion. In other words, if the smallest proportion in which B combines with A, be denoted by 10, A may combine with twice 10 of B, or with three times 10, and so on; but with no intermediate quantities. There cannot be a more striking instance of this law than that above mentioned, of the compounds of manga¬ nese with oxygen ; in which the oxygen of the three last compounds may be observed to be a multiplication of that of the first (14) by the numbers 2, 3, and 4. Examples, in¬ deed, of this kind have, of late, so much increased in number, that the law of simple multiples bids fair to become universal, with respect at least to chemical compounds, the proportions of which are definite.
Facts of this kind are not only important in themselves, but also on account of the generalizations that have been de¬ duced from them ; for on them Mr. Dalton has founded what may be termed the Atomic Theory of the chemical Constitution of Bodies. Till this theory was proposed, we had no adequate explanation of the uniformity of the proportions of chemical compounds ; or of the nature of the cause which renders, combination, in other proportions, impossible. In this place I shall offer only a brief illustration of the theory ; for in the course of the work I shall have occasion to apply it to the explanation of a variety of phenomena.
Though we appear, when wre effect the chemical union of bodies, to operate on masses , yet it is consistent with the most rational view of the constitution of bodies to believe, that it is only between their ultimate particles , or atoms , that combi¬ nation takes place.. By the term atoms , it has been already
30
CHEMICAL AFFINITY, &C.
CHAP. II,
stated, we are to understand the smallest parts of which bodies are composed. An atom, therefore, must be mechanically indivisible, and of course a fraction of an atom cannot exist. Whether the atoms of different bodies be of the same size, or of different sizes, we have no sufficient evidence. The pro¬ bability is, that the atoms of different bodies are of unequal sizes ; but it cannot be determined whether their sizes bear any regular proportion to their weights. We are equally ignorant of their shape; but it is probable, though not essen¬ tial to the theory, that they are spherical. This, however, requires a little qualification. The atoms of all bodies pro¬ bably consist of a solid corpuscle, forming a nucleus, and of an atmosphere of heat, by which that corpuscle is surrounded ; for absolute contact is never supposed to take place between the atoms of bodies. The figure of a simple atom may rea¬ dily, therefore, be conceived to be spherical. But in com¬ pound atoms, consisting of a single central atom, surrounded by other atoms of a different kind, it is obvious that the figure (contemplating the solid corpuscles only) cannot be spherical ; yet if we include the atmosphere of heat, the figure of a compound atom may be spherical, or some shape ap¬ proaching to a sphere.
Taking for granted that combination takes place between the atoms of bodies only, Mr. Dalton has deduced, from the relative weights in which bodies unite, the relative weights of their ultimate particles, or atoms. When only one combina¬ tion of any two elementary bodies exists, he assumes, unless the contrary can be proved, that its elements are united atom to atom singly. Combinations of this sort he calls binary . But if several compounds can be obtained from the same ele¬ ments, they combine, he supposes, in proportions expressed by some simple multiple of the number of atoms. T he fol¬ lowing table exhibits a view of some of these combinations :
1 atom of A + 1 atom of B = 1 atom of C, binary.
1 atom of A + 2 atoms of B = 1 atom of D, ternary.
2 atoms of A + 1 atom of B = 1 atom of E, ternary.
1 atom of A + 3 atoms of B = 1 atom of F, quaternary.
3 atoms of A 4- 1 atom of B = 1 atom of G, quaternary.
SECT. III.
CHEMICAL AFFINITY, &C.
31
A different classification of atoms has been proposed by Berzelius, viz. into, Istly, elementary atoms ; 2dly, compound atoms. The compound atoms he divides again into three different species, namely, 1st, atoms formed of only two ele¬ mentary substances united, or compound atoms of the first order: 2dly, atoms composed of more than two elementary substances; and these, as they are only found in organic bodies, or bodies obtained by the destruction of organic matter, he calls organic atoms : Sdly, atoms formed by the union of two or more com¬ pound atoms; as for example, the salts. These he calls com - pound atoms of the second order .
If elementary atoms of different kinds were of the same size, the greatest number of the atoms of A that could be com¬ bined with an atom of B would be 1 2 ; for this is the greatest number of spherical bodies that can be arranged in contact with a sphere of the same diameter. But this equality of size, though adopted by Berzelius, is not necessary to the hypo¬ thesis of Mr. Dalton ; and is, indeed, supposed by him not to exist.
As an illustration of the mode in which the weight of the atoms of bodies is determined, let us suppose that any two elementary substances, A and B, form a binary compound ; and that they have been proved experimentally to unite in the proportion, by weight, of 5 of the former to 4 of the latter ; then, since, according to the hypothesis, they unite particle to particle, those numbers will express the relative weights of their atoms. But besides combining atom to atom singly, 1 atom of A may combine with 2 of B, or with 3, 4, &c. Or 1 atom of B may unite with 2 of A, or with 3, 4, &c. When such a series of compounds exists, the relative proportion of their elements ought necessarily, on analysis, to be proved to be 5 of A to 4 of B ; or 5 to (4 -f 4 = ) 8 ; or 5 to (4 + 4 + 4 =) 12, &c. ; or, contrariwise, 4 of B to 5 of A ; or 4 to (5 4- 5 = ) 10; or 4 to (5 -f 5 *f 5 = ) 15. Between these there ought to be no intermediate compounds ; and the existence of any such (as 5 of A to 6 of B, or 4 of B to If of A) would, if clearly established, militate against the hypothesis.
To verify these numbers, it may be proper to examine the
CHEMICAL AFFINITY, &C.
CHAP. II.
xJ JLi
combinations of A and B with some third substance, for ex¬ ample with C. Let us suppose that A and 0 form a binary compound, in which analysis discovers 5 parts of A and 3 of C. Then, if C and B are, also, capable of forming a binary com¬ pound, the relative proportion of its elements ought to be 4 of B to 3 of C ; for these numbers denote the relative weights of their atoms. Now this is precisely the method, by which Mr. Dalton has deduced the relative weights of oxygen, hydrogen, and nitrogen ; the two first from the known composition of water, and the two last from the proportion of the elements of ammonia. Extending the comparison to a variety of other bodies, he has obtained a scale of the relative weights of their atoms.
In several instances, additional evidence is acquired of the accuracy of the weight, assigned to an element, by our ob¬ taining the same number from the investigation of several of its compounds. For example:
1. In water , the hydrogen is to the oxgen as 1 to 7*5.
2. In olejiant gas , the hydrogen is to the carbon as 1 to 5*65.
3. In carbonic oxide the oxygen is to the carbon as 7*5 to 5'(i5.
Whether, therefore, we determine the weight of the atom of carbon, from the proportion in which it combines with hyd rogen, or with oxygen, we arrive at the same number 5*65; an agreement which, as it occurs in various other instances, can scarcely be an accidental coincidence. In a similar man¬ ner, 7*5 is declucible, as representing the atom of oxygen, both from the combination of that base with hydrogen and with carbon; and 1 is inferred to be the relative weight of the atom of hydrogen from the two principal compounds into which it enters.
In selecting the body, w7hich should be assumed as unity, Mr. Dalton has been induced to fix on hydrogen, because it is that body which unites with others in the smallest propor¬ tion. Thus, in water, we have 1 of hydrogen by weight to 7~ of oxygen in ammonia, 1 of hydrogen to 5 of nitrogen ; in carbureted hydrogen, 1 of hydrogen to 5*65 of carbon ; and in sulphureted hydrogen, 1 of hydrogen to 15 of sulphur.
SECT. III.
CHEMICAL AFFINITY, &C.
ss
Taking for granted that all these bodies are binary compounds, we have the following scale of numbers, expressive of the re¬ lative weights of the atoms of their elements :
Hydrogen . . . . . . 1
Oxygen . 7*5
Nitrogen . . . 5*0?
Carbon . 5*65
Sulphur . . . 15*0
Drs. Wollaston and Thomson, and Professor Berzelius, on the other hand, have assumed oxygen as the decimal unit, chiefly with a view to facilitate the estimation of its numerous compounds with other bodies. This, perhaps, is to be regret¬ ted, even though the change may be in some respects eligible, because it is extremely desirable that chemical writers should employ an universal standard of comparison for the weights of the atoms of bodies. It is easy, however, to reduce their numbers to Mr. Dalton’s by the rule of proportion. Thus as 10 (the number of Drs. Wollaston and Thomson for oxygen) is to 1*32 (their number of hydrogen) so very nearly is 7*5 (Mr. Dalton’s number for oxygen) to 1 (his number for hy¬ drogen).
Sir H. Davy has assumed, with Mr. Dalton, the atom of hydrogen as unity ; but that philosopher, and Berzelius also, have modified the theory, by taking for granted that water is a compound of one proportion (atom) of oxygen, and two pro¬ portions (atoms) of hydrogen. This is founded on the fact, that two measures of hydrogen gas and one of oxygen gas, are necessary to form water; and on the supposition, that equal measures of different gases contain equal numbers of atoms. And as, in water, the hydrogen is to the oxygen by weight as 1 to 7*5, two atoms or volumes of hydrogen must, on this hypothesis, weigh 1, and one atom or volume of oxygen 7*5, or if we denote a single atom of hydrogen by 1, we must ex¬ press an atom of oxygen by 15. It is objectionable, however, to this modification of the atomic theory, that it contradicts a fundamental proposition of Mr. Dalton, the consistency of which with mechanical principles he has fully shown ; namely,
vol. t. D
34
CHEMICAL AFFINITY, &C.
CHAP. II,
that when one combination only of two elements exists, as be¬ tween oxygen and hydrogen, it must be presumed, unless the contrary can be proved, to be a binary one.
It is easy to determine, in the manner already explained, the relative weights of the atoms of two elementary bodies, which unite only in one proportion. But when one body unites, in different proportions, with another, it is necessary, in order to ascertain the weight of its atom, that we should know the smallest proportion in which the former combines with the latter. Thus, if we have a body A, 100 parts of which by weight combine with not less than 30 of oxygen, the relative weight of its atom will be to that of oxygen as 100 to 30 ; or, reducing these numbers to their lowest terms, as 25 to 7*5 ; and the number 25 will, therefore, express the relative weight of the atom of A. But if, in the progress of science, it should be found, that 100 parts of A are capable of uniting with 15 parts of oxygen, then the relative weight of the atom of A must be doubled, for, as 100 to 15, so is 50 to 7*5. This example will serve to explain the changes, that have been sometimes made, in assigning the weights of the atoms of certain bodies ; changes, which, it may be observed, always consist either in a multiplication, or division, of the original 'weight, by some simple number.
There are (it must be acknowledged) a few cases, in which one body combines with another in different proportions ; and yet the greater proportions are not multiples of the less, by any entire number. For example, we Iiave two oxides of iron, the first of which consists of 100 iron and about 30 oxygen ; the second of 100 iron and about 45 oxygen. But the num¬ bers 30 and 45 are to each other as 1 to 14. It will, however, render these numbers (1 and 1-|) consistent with the law of simple multiples, if we multiply each of them by 2, which will change them to 2 and 3 ; and if we suppose that there is an oxide of iron (though it has not yet been obtained experi¬ mentally), consisting of 100 iron and 15 oxygen; for the mul¬ tiplication of this last number by 2 and 3, will then give us the known oxides of iron.
In some cases, where we have the apparent anomaly of 1 atom of one substance, united with of another, it has been
SECT. III.
CHEMICAL AFFINITY, &C.
35
proposed, by Dr, Thomson to remove the difficulty, by multiplying both numbers by 2; and by assuming that, in such compounds, we have 2 atoms of the one combined with 3 atoms of the other. Such combinations, it is true, are ex¬ ceptions to a law deduced by Berzelius ; that, in all inorganic compounds , one of the constituents is in the state of a single atom. But they are in no respect inconsistent with the views of Mr. Dalton ; and are, indeed, expressly admitted by him to be compatible with his hypothesis, as well as confirmed by experience f. Thus it will appear, in the sequel, that some of the compounds of nitrogen with oxygen are con¬ stituted in this way.
Several objections have been proposed to the theory of Mr. Dalton ; but, of these, I shall notice only the most important.
1. It has been contended, that we have no evidence, when one combination only of two elements exists, that it must be a binary one ; and that we might equally well suppose it to be a compound of two atoms of the one body, with one atom of the other. In answer to this objection, we may urge the pro¬ bability that when two elementary bodies A and B unite, the most energetic combination will be that in which one atom of A is combined with one atom of B ; for an additional atom of B will introduce a new force, diminishing the attraction of those elements for each other, namely, the mutual repulsion of the atoms of B ; and this repulsion will be the greater, in proportion as we increase the number of the atoms of B.
2dly. It has been said, that, when more than one compound of two elements exist, we have no proof which of them is the binary compound, and which the ternary ; for example, that we might suppose carbonic acid to be a compound of an atom of charcoal and an atom of oxygen, and carbonic oxide to be a compound of an atom of oxygen with two atoms of charcoal. To this objection, however, it is a satisfactory answer, that such a constitution of carbonic acid and carbonic oxide would be directly contradictory of a law of chemical combination, namely, that it is attended, in most cases, with an increase of specific gravity. It would be absurd, therefore, to suppose carbonic acid, which is the heavier body, to be only once com-
d 2
Thomson’s Annals, v. 187.
-f Thomson^ Annals, iii, 174.
36
CHEMICAL AFFINITY, &C.
CHAP. II,
pounded, and carbonic oxide, which is the lighter, to be twice compounded. Moreover, it is universally observed, that of chemical compounds, the most simple are the most difficult to be decomposed ; and this being the case -with carbonic oxide, we may naturally suppose it to be more simple than carbonic acid.
3dly. It has been remarked, that instead of supposing wa¬ ter to consist of an atom of oxygen united with an atom of hydrogen, and that the atom of the former is 74- times heavier than that of the latter, we might, with equal probability, con¬ clude that, in water, we have 74- times more atoms in number of oxygen than of hydrogen. But this, if admitted, would involve the absurdity, that in a mixture of hydrogen and oxygen gases, so contrived that the ultimate atoms of each should be in equal number, seven atoms of oxygen should desert all the proximate atoms of hydrogen, in order to unite with one at a distance, for which they must necessarily have a less affinity. In this case, a less force must overcome a greater; and, finally, only a small number of the atoms of hydrogen would be engaged by the atoms of oxygen, the rest remaining in a state of freedom.
It would be claiming too much, however, for the theory of Mr. Dalton to assert that, in its present state, it is to be con¬ sidered as fully established in all its details. In the further progress of chemical discovery, it is probable that it will re¬ ceive considerable modifications, and that the relative weights of the atoms of bodies will, in many cases, be essentially changed. The instances, in which the theory agrees with the results of analysis, are already too numerous to allow them to be considered as accidental coincidences ; and no phenomena have hitherto been shown to be irreconcileable with the hypo¬ thesis. Its value and importance, if confirmed by the acces¬ sion of new facts, will be scarcely less felt as a guide to fur¬ ther investigations into the constitution of bodies, than as a test of the accuracy of our present knowledge ; and the uni¬ versality of its application to chemical phenomena will be scarcely inferior to that of the lawr of gravitation in explaining the facts of natural philosophy*.
* A perspicuous and able statement of the atomic theory, published by Mr. Ewart, in the sixth volume of Thomson's Annals, deserves the reader’s perusal.
SECT. III.
CHEMICAL AFFINITY, & C.
37
A modification of the law of definite proportions, so far as respects aeriform bodies, has been proposed by Gay Lussac, namely, that they combine in proportions determinable not by weight but by volume , the ratios being 1 measure of A to 1 of B, or 1 to 2, or 1 to 3, &c. Water, for example, re¬ sults from the union of 2 volumes of hydrogen with 1 volume of oxygen ; muriate of ammonia from 1 volume of muriatic acid gas + 1 of ammonia ; nitrous gas from 1 measure of oxygen + 1 of nitrogen ; nitrous oxide from 1 oxygen + 2 ni¬ trogen ; nitrous acid from 2 oxygen + 1 nitrogen. In some instances, as in that of water, this law is not inconsistent with the atomic theory; but in other instances, it cannot be recon¬ ciled with the relative weights assigned to the atoms of certain elementary bodies. In nitrous gas, for example, which Mr. Dalton conceives to be formed by the union of 1 atom of oxygen + 1 atom of nitrogen, equal volumes of those gases would give for the relative weights of oxygen and nitrogen, numbers differing widely from those derived by other methods. The two hypotheses of atoms and of volumes cannot, therefore, both be true ; and from some well ascertained exceptions to the latter, it appears to me that the theory of volumes will scarcely be found tenable.
Before dismissing the consideration of the proportions in which bodies combine, it will be proper to notice a few gene¬ ral principles, which, though they are connected with the atomic theory, have been derived from experience.
1. When we have ascertained the proportion in which any two or more bodies ABC &c. of one class neutralize another body X of a different class, it will be found that the same re¬ lative proportions of A B C See. will be required to neutralize any other body of the same class as X. Thus, since 100 parts of real sulphuric acid, and 68 (omitting fractions) of muriatic acid neutralize 118 of potash, and since 100 of sulphuric acid neutralize 71 of lime, we may infer that 68 of muriatic acid will neutralize the same quantity (71) of lime. The great importance of this law will readily be perceived, not only as it enables us to anticipate, but also to correct, the results of analysis.
2dly. If the quantities of two bodies, A and B, that are m>
38
CHEMICAL AFFINITY, &C.
CHAP. II.
cessary to saturate a given weight of a third body X, be re¬ presented by q and r, these quantities may be called equiva¬ lents. Thus, in the example above cited, 100 parts of sul¬ phuric acid and 68 of muriatic acid, are equivalents of each other. A Table of Equivalents, which will be found extremely useful in various calculations, will be given in the Appendix. By adapting a table of this sort to a moveable scede, on the principle of Gunter’s sliding rule, Dr. Wollaston has lately constructed an instrument, called the Logornetric Scale of Che¬ mical Equivalents , which is capable of solving, with great facility, a number of problems, interesting both to the scien¬ tific and practical chemist*.
SECTION IV.
Of Elective Affinity.
An important law of affinity, which is the basis of almost all chemical theory, is, that one body has not the same force of affinity towards a number of others, but attracts them un- equally. Thus A will combine with B in preference to C, even when these two bodies are presented to it under equally favourable circumstances. Or, when A is united with C, the application of B will detach A from C, and we shall have a new compound consisting of A and B, C being set at liberty. Such cases are examples of what is termed in chemistry simple decomposition , by which it is to be understood that a body acts upon a compound of two ingredients, and unites with one of its constituents, leaving the other at liberty. And as the forces of affinity of one body to a number of others vary, this body has been metaphorically represented as making an elect¬ ion ; and the affinity has been called single elective affinity . Thus if to the muriate of lime, consisting of lime and mu¬ riatic acid, we add potash, the muriatic acid exerts a stronger
* This instrument may be had, with printed instructions for its use, of Mr. Carey, 182, Strand, London; and its cost is so trifling, that I consider a plate of it to be quite unnecessary.
ECT. IV.
CHEMICAL AFFINITY, &C.
39
elective affinity for the potash than for the lime ; and the lime falls down in the state of a powder, or is precipitated. Of facts of this kind a great variety have been comprehended in the form of tables, the first idea of which seems to have oc¬ curred nearly a century ago, to Geoffroy, a French chemist. The substance, whose affinities are to be expressed, is placed at the head of a column, and is separated from the rest by a horizontal line. Beneath this line are arranged the bodies, with which it is capable of combining, in the order of their respective forces of affinity ; the substance which it attracts most strongly being placed nearest to it, and that, for which it has the least affinity, at the bottom of the column. The affinities of muriatic acid, for example, are exhibited by the following plan : —
MURIATIC ACID.
/
Barytes,
Potash,
Soda,
Lime,
Ammonia,
Magnesia,
&c. &c.
Simple decompositions may be expressed also by another form, contrived by Bergman. Thus the following scheme il¬ lustrates the decomposition of muriate of magnesia by potash;-*
Muriate of Potash.
Muriate i Muriatic acid. Potash.
°*' . < Water at 60°.
Magnesia, i
& # Magnesia.
— """v — — '■ — ■ "
Magnesia.
The original compound (muriate of magnesia) is placed on the outside and to the left of the vertical bracket. The in¬ cluded space contains the original principles of the compound, and also the body which is added to produce decomposition. Above and below the horizontal lines are placed the results of their action. The point of the lower horizontal line being
40
CHEMICAL AFFINITY, &C.
CHAP. II.
turned downwards, denotes that the magnesia falls down or is precipitated; and the upper line, being perfectly straight, shows, that the muriate of potash remains in solution. If both the bodies had remained in solution, they would both have been placed above the upper line ; or, if both had been pre¬ cipitated, beneath the lower one. If either one or both had escaped in a volatile form, this would have been expressed by placing the volatilized substance above the diagram, and turn¬ ing upwards the middle of the upper horizontal line. But since decompositions vary under different circumstances, it is necessary to denote, by the proper addition to the scheme, that the bodies are dissolved in water of the tem¬ perature of 60°.
No chemical facts can appear, on first view, more simple or intelligible, than those which are explained by the operation of single elective affinity. It will be found, however, on a more minute examination, that this force, abstractedly con¬ sidered, is only one of several causes which are concerned in chemical decompositions, and that its action is modified, and sometimes even subverted, by counteracting forces.
SECTION V.
Of the Causes which modify the Action of Chemical Affinity.
The order of decomposition is not, as might be inferred from the law of elective affinity, invariable ; but, in certain cases, may even be reversed. Thus though A may attract B more strongly than either A or B is attracted by C, yet, under some circumstances, C may be employed to decompose par¬ tially the compound A B. Again, if we mix together A B and C, using the two first in the proportions required to neutralize each other, it will be found that A and B have not combined to the exclusion of C, but that we have a compound of B with A, and another of B with C, in proportions regulated by the quantities of A and C, which have been employed. Facts of this kind have been long known to chemists. It had been as¬ certained, for example, before the time of Bergman, that sub
SECT. V.
CHEMICAL AFFINITY, &C.
41
phate of potash, a salt composed of sulphuric acid and potash, is partly decompounded by nitric acid, although the nitric has a weaker affinity than the sulphuric acid for that alkali. Ex¬ amples of the same kind have since been multiplied by Ber- thollet, who has asserted that in the following, as well as in other cases, a substance possessing a weaker attraction, dis¬ places another having a stronger, for a third body # :
1. Potash separates sulphuric acid from barytes.
2. Lime separates sulphuric acid from potash.
3. Potash separates oxalic acid from lime.
4. Nitric acid separates lime from oxalic acid.
5. Potash separates phosphoric acid from lime.
6. Potash separates carbonic acid from lime.
7. Soda separates sulphuric acid from potash.
These facts, and a variety of similar ones, are to be explained, according to the viewrs of Berth oil et, on the following prin¬ ciples : —
1. When two substances are opposed to each other with re¬ spect to a third, as in the foregoing examples, they may be considered as antagonist forces ; and they share the third body between them in proportion to the intensity of their action* But this intensity, according to Berth ol let, depends not only on the energy of the affinities , but on the quantities of the two bodies opposed to each other. Hence a larger quantity of one of the substances may compensate a weaker affinity, and the reverse. To the absolute weight of a body, multiplied by the degree of its affinity, he has given the name of mass, a term in some degree objectionable from the different mean¬ ing which is affixed to it in mechanical philosophy. As an illustration, let us suppose (what is not accurate in point of fact) that the affinity of barytes for muriatic acid is twice as strong as that of potash, or that these affinities are respectively denoted by the numbers 4 and 2. In this case the same mass will result from 4 parts of barytes as from 8 of potash ; be-
* In each of the examples given in the Table, the body, first mentioned, decomposes a compound of the second and third, although its attraction for the second is inferior to that of the third.
42
CHEMICAL AFFINITY, &C.
CHAP. II.
cause the same product (16) is obtained in each instance, by multiplying the number indicating the affinity into that de¬ noting the quantity ; for 4 (the affinity of barytes) multiplied by 4, (the quantity assumed in this example) is equal to 16 ; and 2 (the affinity of potash) multiplied by 8 (its quantity) is also equal to 16. In this case, therefore, to divide equally a portion of muriatic acid between barytes and potash, these bodies should be employed in the proportion of 2 of the former to 4 of the latter.
The influence of quantity explains also the difficulty which is observed in effecting, in any instance, the total decomposi¬ tion of a compound of two principles by means of a third. The immediate effect of a third body C, when added to a compound A Ij, is to abstract from B a portion of the substance A ; and consequently a portion of B is set at liberty, the attrac¬ tion of which for A is opposed to that of the uncombined part of C. The farther this decomposition is carried, the greater will be the proportion of B, which is brought into an uncombined state ; and the more powerfully will it oppose any farther tendency of C to detach the substance A. At a cer¬ tain point, the affinities of B and C for A will be exactly ba¬ lanced, and the decomposition will proceed no farther. In a few cases, it is acknowledged by Berthollet, a third body se¬ parates the whole of one of the principles of a compound ; but this he supposes to happen in consequence of the agency of other extraneous forces, the nature of which remains to be pointed out.
2dlv. Cohesion is a force, the influence of which over the chemical union of bodies has already been explained in a former section ; and other illustrations of its interference will be given, when we consider the subject of the limitations to chemical combination.
Sdly. Insolubility is another force, which essentially modifies the exertion of affinity. It is to be considered, indeed, merely as the result of cohesion, with respect to the liquid in which
the effect takes place.
When a soluble substance and an insoluble one are pre¬ sented, at the same time, to a third, for which they have nearly an equal affinity, the soluble body is brought into the
SECT, V.
CHEMICAL AFFINITY, &C.
43
sphere of action with great advantages over its antagonist. Its cohesion at the outset is but little, and by solution is reduced almost to nothing ; while that of the insoluble body remains the same. The whole of the soluble substance also exerts its affinity at once; while a part only of the insoluble one can oppose its force. Hence the soluble substance may prevail, and may attach to itself the greatest proportion of the third body, even though it has a weaker affinity than the insoluble one to the subject of combination.
Insolubility, however, under certain circumstances, is a force which turns the balance in favour of the affinity of one body when opposed to the affinity of another. For example, if to the soluble compound, sulphate of soda, we add barytes, the new compound, sulphate of barytes, is precipitated the instant it is formed: and being removed from the sphere of action, the soda can exert no effect upon it by its greater quantity or mass. For the same reason, when soda is added to sulphate of barytes, the sulphate is protected from decom¬ position both by its insolubility and by its cohesion.
These facts sufficiently prove that the order of precipitation, which was formerly assumed as the basis of tables of elective affinity, can no longer be considered as an accurate measure of that force ; and that the body, which is precipitated, may, in some cases, be superior in affinity to the one which has caused precipitation. In these cases, a trifling superiority in affinity may be more than counterbalanced by the cohesive force, which causes insolubility.
4thly, Great specific gravity is a force, which must concur with insolubility or cohesion in originally impeding combina¬ tion; and when chemical union has taken place, it must come in aid of affinity, by removing the new compound from the sphere of action. It is scarcely necessary to enlarge on the operation of a force, the nature of which must be so obvious.
5thly. Elasticity. Cohesion, it has already been stated, may prove an impediment to combination ; and on the other hand, it is possible that the particles of bodies may be sepa¬ rated so widely, as to be removed out of the sphere of their mutual attraction. Such appears to be the fact with regard to a class of bodies called airs or gases. The bases of several
3
44
CHEMICAL AFFINITY, &C,
CHAP. IT.
of these have powerful attractions for the bases of others, and for various liquids, and yet they do not combine on simple admixture, but strong mechanical pressure brings their par¬ ticles sufficiently near, to be within the influence of their mutual attraction, and combination immediately ensues.
Again if two bodies, one of which has an elastic and the other a liquid form, be presented at the same time to a solid, for which they have both an affinity, the solid will unite with the liquid in preference to the gas. Or if we add to the com¬ pound of an elastic substance with an inelastic one, a third body also inelastic, the two latter combine to the exclusion of the elastic body. For example, if to the compound of pot¬ ash and carbonic acid we add sulphuric acid, the latter acid, acting both by its affinity and its quantity, disengages a por¬ tion of carbonic acid. This, by its elasticity, is removed from the sphere of action, and presents no obstacle to the farther operation of the sulphuric acid. Hence elastic bodies act only by their affinity ; whereas liquids act both by their affi¬ nity and quantity conjoined. And though the affinity of the liquid, abstractedly considered, may be inferior to the affinity of the elastic body, yet, united with quantity, it prevails. In the above instances, the whole of the elastic acid may be ex¬ pelled by the fixed acid ; whereas, as it has already been ob¬ served, decomposition is incomplete, if the substance which is liberated remain within the sphere of action.
6thly, Efflorescence is a circumstance which occasionally in¬ fluences the exertion of affinity; but this is only of very rare occurrence. . The simplest example of it is that of lime, and muriate of soda. When a paste composed of these two sub¬ stances with a great excess of lime, is exposed, in a moist state, to the air, the lime, acting by its quantity, disengages soda from the common salt, which appears in a dry form, on the outer surface of the paste, united with carbonic acid ab¬ sorbed from the atmosphere. In this case the soda, which is separated, being removed from contiguity with the interior part of the mass, presents no obstacle to the farther action of the lime, and the decomposition is carried farther than it would have been, had no such removal happened.
7thly. The influence of temperature over chemical affinity
SECT. V.
CHEMICAL AFFINITY, &C.
45
is extremely extensive and important ; but at present a very general statement only of its effects is required. In some cases an increased temperature acts in promoting, and at others in impeding, chemical combination : and it materially affects also the order of decompositions.
An increased temperature promotes chemical union by diminishing or overcoming cohesion. Thus metals unite by fusion, and several salts are more soluble in hot that in cold water. Whenever heat is an obstacle to combination, it pro¬ duces its effect by increasing elasticity. Hence water absorbs a less proportion of gas at a high than at a low temperature. A reduction of the temperature of elastic bodies, by lessening their elasticity, facilitates their union with other substances. In certain cases, an increased temperature has the combined effects of diminishing cohesion and increasing elasticity. When sulphur is exposed to oxygen gas, no combination ensues, until the sulphur is heated ; and though the elasticity of the gas is thus increased, yet the diminution of cohesion of the solid is more than proportionate, and chemical union ensues between the two bodies.
8thly. The electrical state of bodies has a most important influence over their chemical union. This, however, is a subject, of which it would be difficult to offer a general view", and for its full development, I refer to a subsequent chapter on Electro-chemistry .
9thly. Mechanical pressure is another force, which has con¬ siderable influence over chemical affinity. WTith respect to solid bodies, its agency is not frequent ; but we have unequi¬ vocal examples of its operation in cases, where detonation is produced by concussion. The effects of pressure are chiefly manifested, in producing the combination of aeriform bodies either with solids, with liquids, or with each other ; and in preserving combinations, which have been already formed, under circumstances tending to disunite them. Chalk, for example, is a compound of lime and carbonic acid ; and these bodies, by the simple application of an intense heat, are separable from each other ; but, under strong pressure, a heat may be applied sufficient to melt the chalk, without ex¬ pelling the carbonic acid. It is this principle, (of the in-
46
CHEMICAL AFFINITY, &C.
CHAP. II.
fiuence of pressure in opposing chemical decomposition,) that is the foundation of Dr. Hutton’s ingenious Theory of the Earth.
Such are the most important circumstances, that modify the exertion of cheminal affinity. Of their influence, suf¬ ficient illustrations have been given to prove, that in every case of combination and decomposition, we are not to con¬ sider the force of affinity abstractedly ; but are to take into account the agency of other powers, as cohesion, quantity, insolubility, elasticity, efflorescence, and temperature. By the action of these extraneous powers, Berthollet has endeavoured to explain certain facts which are not easily understood on any other principle. Of these the most important are, lstly, the establishment of proportions in chemical compounds; and 2dly, the modification produced in the affinities of bodies by chemical union.
1. Independently of these extraneous forces, Berthollet imagines that there are no limits to combination, or that two bodies, which are now susceptiple of union only in one or in few' proportions, might, if these forces were annihilated, be united in every proportion. The causes which he has as¬ signed, as chiefly regulating proportion, are cohesion and elasticity. To take one of the simplest cases, the proportion, in which a salt can be combined with water, depends on the balance between the chemical affinity of the bodies for each other, and the cohesive attraction of the salt. In this case, then, cohesion is the limiting power. As an example of the influence of this force when more energetic affinities are ex¬ erted, if wre add to diluted sulphuric acid a solution of barytes, a compound is formed, consisting of sulphuric acid and barytes, which, in consequence of its great insolubility or co¬ hesion, is instantly removed from contact with the redundant acid, and with established proportions.
The agency of elasticity in limiting proportion, may be exemplified by the combination of hydrogen and oxygen. If a mixture of the two gases be inflamed, the new compound, water, is immediately separated, from what is superfluous ol both ingredients, by its superior density. In other instances, the bases of aeriform substances are combined in various
SECT. V.
CHEMICAL AFFINITY, &C.
47
proportions, and in such examples, there are several terms of greatest condensation, as in the case of oxygen and ni¬ trogen.
2. Another important part of the theory of Rerthollet is, that the affinities of a compound are not newly acquired ; but are merely the modified affinities ol its constituents, the action of which, in their separate state, was counteracted by the prevalence of opposing forces. By combination, these forces are so far overcome, that the affinities of the constituents are enabled to exert themselves.
The action of different affinities existing in one compound, Berthollet terms resulting affinities , while the individual affi¬ nities of the constituents he calls elementary affinities. Thus nitric acid acts on potash by an affinity, which results from those of oxygen and azote for potash. And as ail affinity is mutual, the term resulting affinity is applied, also, to that force, with which a simple body acts on a compound ; to the affinity for example, which any simple body may exert on nitric acid. A simple body, indeed, may exert towards a compound both an elementary and resulting affinity. If the elementary affi¬ nity prevails, it will unite only with one of the principles of the compound, as when a simple body, by its affinity for oxygen, decomposes nitric acid, and liberates its nitrogen in a separate form. If the resulting affinity be predominant, the simple body will unite with the whole compound without effecting any disunion of its elements.
From these views it may be inferred, that wre are not, in any case, to deny the existence of an affinity between two bodies, merely because they do not combine when presented to each other ; for an affinity may exist, but may be suppressed by the prevalence of opposing forces. According to the doctrine of Berthollet, affinity is a force exerted by every body towards every other; even though not made apparent by any effect. On this principle, we are able to explain certain phenomena, which are wholly unintelligible on any other, and especially those which have been referred to disposing affinity . The action of sulphuret of potash, for example, on oxygen gas, has been ascribed to the disposing affinity of potash for sul¬ phuric acid. This, however, is ascribing an affinity to a
48
CHEMICAL AFFINITY, &C.
CHAP. II.
compound, before that compound has existence. It is much more probable, that besides the diminished cohesion of the sulphur, the affinity of potash for oxygen has some share in producing the combination. On this principle the united affinities of the potash and sulphur for oxygen (in other words the resulting affinities of the sulphuret of potash) are the efficient causes of chemical union. This explanation, at least, does not, like the theory of disposing affinities, involve an absurdity.
The theory of Berthollet, however, which promised, on its first development, to form a new era in chemical philosophy, has lost much of its probability, by the subsequent progress of the science. It is directly, indeed, at variance with the doctrine of definite proportions, which every day gathers strength by the accumulation of new and well established facts. It is liable, moreover, to the following objections.
1st. It has been shown by Professor PfafF, of Kiell *, that, in various cases, where two acids are brought into contact with one base, the base unites with one acid, to the entire exclusion of the other. When, for example, to a given weight of lime, quantities of sulphuric and tartaric acids are put, either of which would exactly saturate the lime, the sul¬ phuric acid unites with the lime, to the entire exclusion of the tartaric. The same evidence of a superior affinity of the sulphuric acid over that of the oxalic is obtained, by placing those acids in contact with as much oxide of lead, as would exactly saturate either of them. Again, comparing the action of two bases on one acid, the same law is found to hold good: for when potash and magnesia are mixed with just as much sulphuric acid, as is required to saturate either of them, the potash seizes the whole of the acid, and no part of it unites with the magnesia. Nor can these effects be explained by any of those extraneous forces, which Berthollet supposes, in all cases, to regulate chemical combination ; or by any principle, but a stronger affinity of sulphuric acid, than of tartaric or oxalic acid, for the different bases; and of potash, than of magnesia, for the same acid.
*
77 Ann. de Chim. p. 259.
SECT. V.
CHEMICAL AFFINITY, &C,
2dly. Some of the eases, before quoted from Berthollet, to show the reciprocal displacement of two bodies by each other from a third (it has been justly observed), are examples, not of single elective affinity, in which three bodies only are con¬ cerned; but of complex affinity, in which the attractions of four bodies are brought into action. In the first case, for example, there is reason to believe, that sulphuric acid is dis- placed from barytes, not by pure potash, but by potash which has absorbed carbonic acid from the atmosphere.
3dly. In other cases, the consideration of the affinities of two bodies A and B, for a third C, is complicated with thi* circumstance, that the neutral compound of A and B has an affinity for a farther portion of one of its ingredients. If then C be brought into contact with the compound A B, we have, acting at the same moment, the affinity of C for A, which partly decomposes the compound A B ; and the affinity of the undecomposed part of A B, for that portion of B which is set at liberty. For instance, when nitric acid acts on sulphate of potash, some nitrate of potash is formed ; and the sulphuric acid, which is set at liberty, uniting with the undecomposed sulphate of potash, composes a new salt, consisting of sulphate of potash with an excess of sulphuric acid.
4thly. It is a strong objection to the theory of Berthollet that, in some cases, decompositions happen, which, according to his views, ought not to take place; and that in others, de¬ compositions do not ensue, which the theory would have led us to have anticipated.
5thly. The theory is objectionable, inasmuch as, in several instances, properties are supposed to operate, before the bodies exist, to which those properties are attributed. It is incon¬ ceivable, for instance, that the cohesion, or insolubility, of sulphate of barytes, can have any share in producing the de¬ composition of sulphate of potash by that earth ; for the inso¬ lubility of sulphate of barytes can have no agency, till that compound is formed; which is the very effect to be explained.
Notwithstanding these objections to the theory of Berthol¬ let, when carried so far as has been done by its author, in the explanation of chemical phenomena, it must still be admitted that the extraneous forces, pointed out by that acute phiioso-
VOL. I. E
5a
CHEMICAL AFFINITY, &C.
CHAP. II.
pher, have great influence in modifying the effects of chemical affinity. But these forces are entitled only to be considered as secondary causes ; and not as determining combinations or decompositions, nor as regulating the proportions in which bodies unite, independently of the superior force of chemical affinity.
SECTION VL
Of the Estimation of the Forces of Affinity .
The affinities of one body for a number of others are not all of the same degree of force. This is all that the present state of our knowledge authorizes us to affirm; for we are ignorant how much the affinity of one body for another is superior to that of a third. The determination of the precise forces of affinity would be an important step in chemical phi¬ losophy : for its phenomena would then be reduced to calcu¬ lation ; and we should be enabled to anticipate the results of experiment. That the force of chemical affinity must be pro¬ digiously great, is evident from its effect in preserving the com¬ bination of water with some bodies (the alkalies for instance) when exposed to a violent heat ; notwithstanding its great ex¬ pansive force, and though water is not essential to the consti¬ tution of those bodies.
The observed order of decomposition, it has already been stated, does not enable us to assign the order of the forces of affinity ; because, in all decompositions, other forces are con¬ cerned. We are, therefore, obliged to seek some other method of determining the problem. Of these several have been pro¬ posed.
When the surface of one body is brought into contact with another surface of the same kind, as when the smooth surfaces of a divided leaden bullet are pressed together, they adhere by the force of cohesion, their particles being all of the same kind . But when the surfaces of different bodies are thus brought into apparent contact, it is reasonable to suppose- that their adhesion arises from chemical affinity, because their particles are of different kinds, Guyton proposed, therefore*
SECT. VI.
CHEMICAL AFFINITY, &C.
51
the comparative force, with which different surfaces adhere, as a competent measure of chemical affinity. His experiments were made on plates of different metals, of precisely the same size and form, suspended by their centres from the arm of a sensible balance. The lower surfaces of these plates were successively brought into contact with mercury, which was changed for each experiment, and the weight was observed, which it was necessary to add to the opposite scale, in order to detach the several metals. Those which required the largest weight were inferred to have the greatest affinity; and it is remarkable, that the order of affinities, as determined in this way, correspond with the affinities as ascertained by other methods. The following were the results :
Gold adhered to mercury with a force of . . . . 446 grains.
Silver . . . . . 429
Tin . . . . . . . . . 418
Lead . 397
Bismuth . . . 372
Zinc . 204
Copper . ...» . 142
Antimony . 126
Iron . 115
Cobalt . 8
This method, it must be obvious, is of too limited applica¬ tion to be of much utility ; for few bodies have the mechanical conditions, which can enable us to subject them to such a test. How, for example, could the affinities of acids for alkalies be examined on this principle? It may be doubted, also, whe¬ ther in the cases to which it may be applied, it does not mea¬ sure the facility of combination, rather than the actual force of affinity.
To determine the absolute forces of affinity, which one body exerts towards a number of others, Mr. Kirwan has proposed the quantity of each which is required to produce neutrali¬ zation, in other words, its equivalent. This he has ascertained by experiment in a great variety of instances, a few of which are contained in the following tables; the numbers being altered, to accommodate them to recent discoveries.
E 2
52
CHEMICAL AFFINITY, &C.
CHAP. II.
100 Parts of
SULPHURIC ACID require for Neutralization
r~~~ — “ — - * - — ■ — - >
194- parts of barytes.
138 .... of strontites.
118 .... of potash.
78*2 .... of soda.
71 .... of lime.
49*2 .... of magnesia.
43 .... of ammonia.
100 Parts of potash require
115 of nitric acid.
93 of carbonic acid. 84*5 of sulphuric.
58 of muriatic.
In judging of the affinities of the same acid for different bases, Mr. Kirwan assumed that they are represented by the numbers indicating the quantities of each base required for neutralization. Thus, because 100 parts of sulphuric acid neutralize 194 of barytes, and 118 of potash, the affinity of the former is superior to that of the latter in the proportion of 194 to 118. So far the inference corresponds with the order of decomposition; for barytes takes sulphuric acid from potash. But if we examine the affinities of potash, as re¬ presented in the second table, we shall find that, on this principle, they are directly contradictory to fact. Thus the affinity of sulphuric acid should be inferior to that of the car¬ bonic ; whereas it is well known that the former displaces the latter from all its combinations. Mr. Kirwan was, therefore, driven to the necessity of establishing a precisely opposite rule in determining the affinities of different acids for the same base, and of assuming that they are inversely proportionate to the affinity of the saturating acid. Thus the affinity of carbonic acid for potash would be represented by 84*5, and that of sul¬ phuric acid 93. This, however, involves a contradiction; since it is implied that a stronger affinity, in one instance, re¬ quires a greater quantity of the saturating principle, as in the relation of barytes and potash to sulphuric acid ; and that, in the other, it requires a less quantity, as in the instance of the sulphuric and carbonic acids with respect to potash.
Since neutralization is an effect of chemical affinity, which must in all cases bear a proportion to its cause, it has been assumed by Berthollet, that the substance which, in the
1
SECT. VII. CHEMICAL AFFINITY, &C. 5$
smallest quantity , neutralizes another, is the one possessing the strongest affinity. On this principle the affinities of sul¬ phuric acid for different bases, will be exactly the reverse of the order established by Mr. Kirwan; and to that order, which would have been assigned from observed decomposi¬ tions. Thus ammonia will have a stronger affinity for sul¬ phuric acid, than any of the substances which are placed above in the table ; though it is separated, by each of these, from its union with that acid.
It is in the extraneous forces, which have been enumerated as influencing chemical affinity, that we are to seek, according to Berthollet, for the explanation of this apparent anomaly, and especially in the forces of cohesion and elasticity® The elasticity of ammonia, for example, turns the balance in favour of magnesia, lime, &c. There is an obvious difficulty, how¬ ever, in the application of the theory. For as the elasticity of ammonia is suppressed by its combination with sulphuric acid, what, it may be asked, but a superior affinity for sul¬ phuric acid, existing in the substances which stand above am¬ monia in the table, can occasion the first commencement of decomposition ? The problem, therefore, of determining the absolute forces of affinity can scarcely be admitted to be solved. Even if it were, we should not be able to predict the order of decomposition, unless the modifying forces of cohesion, elas¬ ticity, &c. could be at the same time subjected to precise ad¬ measurement. Until both these objects are accomplished, the results of chemistry can in no case be obtained by calculation, but the science must remain a collection of general principles^ derived from experiment and induction.
SECTION VIL Of Complex Affinity .
Under the more general name of complex affinity , Berthol¬ let includes that which has hitherto been considered as pro¬ duced by the action of four affinities, and which has com¬ monly been denominated double elective affinity. It frequently happens that the compound of two principles cannot be de-
CHEMICAL AFFINITY, &C.
CHAP. II,
54?
stroyed either by a third or a fourth separately applied ; but if the third and fourth* be combined, and placed in contact with the former compound, a decomposition, or a change of principles will ensue. Thus when lime water is added to a solution of the sulphate of soda, no decomposition happens, because the sulphuric acid attracts soda more strongly than it attracts lime. If the muriatic acid be applied to the same compound, still its principles remain undisturbed, because the sulphuric acid attracts soda more strongly than the muriatic. But if the lime and muriatic acid, previously combined, be mixed with the sulphate of soda, a double decomposition is effected. The lime, quitting the muriatic acid, unites with the sulphuric ; and the soda, being separated from the sul¬ phuric acid, combines with the muriatic. These decompo¬ sitions are rendered more intelligible by the following diagram, contrived by Bergman.
Muriate of Soda
Sulphate
of
Soda
V
Soda
78 }>
115
Muriatic acid
< 10'1 >
Su!phc acid 71
Lime
Sulphate of Lime
Muriate
of
Lime
On the outside of the vertical brackets are placed the ori¬ ginal compounds ; and above and below the diagram, the new compounds. The upper line, being straight, indicates that the muriate of soda remains in solution ; and the middle of the lower line, being directed downwards, that the sulphate of lime is precipitated.
In all cases similar to the foregoing, Mr. Kirwan Conceives that we may trace the operation of two distinct series of affini¬ ties. i he affinities tending to preserve the original compounds (which in the above example are those between sulphuric acid and soda, and between muriatic acid and lime), he terms the quiescent affinities; because they resist any change of composi¬ tion. On the other hand the affinities, which tend to disunite
SECT. VII. CHEMICAL AFFINITY, &C. 5B
the original compounds and to produce new ones (such as those between muriatic acid and soda, and between sulphuric acid and lime), he terms divellent affinities. In order that an effect may be produced, the divellent affinities must necessarily be superior to the quiescent. Now, assuming the numbers in Mr. Kirwan’s tables to express accurately the forces of affini¬ ties, the double exchange of principles, which happens in the preceding instance, is readily explained. Thus the quiescent affinities are
Those of lime to muriatic acid = 104 of soda to sulphuric acid = 78
182
The divellent affinities, opposed to these, consist of
The affinity of soda to muriatic acid = 115
lime to sulphuric acid = 71
186
The original compound, therefore, is preserved by a force equivalent to 182, and the tendencies to produce new com* pounds are represented by the number 186. The divellent affinities are, therefore, predominant.
The theory of quiescent and divellent affinities, however, though highly attractive from its simplicity, and from the facility with which it solves certain phenomena, is completely defective in the explanation of others. For example, sulphate of potash is decomposed by muriate of barytes. Yet, esti¬ mating in the above manner the quiescent and divellent affi¬ nities, an exchange of principles ought not to ensue. The affinities tending to preserve the original compound, are those of sulphuric acid for potash = 118, and of muriatic acid for barytes = 285. The divellent affinities are that of muriatic acid for potash = 174 + that of sulphuric acid for barytes = 194. The quiescent affinities then are 118 + 285 = 403, and the divellent 174 + 194 =.368. This leaves a balance of 35 in favour of the quiescent affinities ; and yet decomposition ensues, when the two compounds are brought into contact.
56
CHEMICAL AFFINITY, &C.
CHAP. II.
It must be acknowledged that the numbers, assumed by Mr. Kirwan, do not correspond with the actual forces of affinity. But even if they are taken according to the principle assumed by Berthollet, they will not be found universally applicable. The reason of this is, that the phenomena produced by com¬ plex affinity, like those occasioned by simple affinity, are ma¬ terially influenced by the extraneous forces of cohesion, quan¬ tity, elasticity, temperature, &c. The effect of quantity is shown by the fact, that if two salts be mixed together in cer¬ tain proportions, decomposition will ensue, but not if mixed in other proportions. Thus from the mingled solutions of two parts of muriate of lime and one of nitrate of potash, wre obtain muriate of potash ; but not from equal weights of the two salts. Insolubility, or precipitation, has also a consider¬ able influence on the result. When this occurs, the influence of quantity is destroyed, as in the case of sulphate of potash and muriate of barytes. Elasticity, and an increased tempe¬ rature (which operates by increasing elasticity), and the re¬ verse of this, or a greatly diminished temperature, have also a powerful influence in promoting the action of complex affi¬ nities. Thus of four principles, two of which are volatile and two fixed, the two which are volatile will be disposed to unite, in preference to combining with either of those which are fixed. The nature of the fluid, in which salts are dissolved, has also an important influence on their ten¬ dency to mutual decomposition*. Thus changes take place In the midst of an alcoholic medium, which do not hap¬ pen to the same bodies dissolved in water t. We have even Instances, in which though a compound A B decomposes an¬ other compound C D, A uniting with C, and B with D, yet (which could not have been expected a priori ) the compound A C is reciprocally decomposed by D B, and the original compounds A B and C D are regenerated if. Hence the phe¬ nomena of complex decomposition concur with those of a more simple kind, in proving that affinity is not an uniform force, but is materially influenced by various modifying cir-
* Ann. de Chim. et Phys. iv. 366. f Dr. Murray on Sea Water: | See the sect, on Sulphate of Barytes.
SECT. VII.
CHEMICAL AFFINITY, &C.
57
cumstances ; and that we cannot confidently anticipate results, from comparing the numerical expressions of quiescent and divellent affinities.
One great obstacle to the construction of tables, capable of representing the forces of affinity, is the difficulty of ascer¬ taining, with precision, the quantities of bodies required for neutralization. Notwithstanding all the care employed by Mr. Kirwan, considerable errors appear to have crept into the results of his experiments. This will sufficiently appear, when they are examined by a test, originally proposed by Guyton. It must be obvious that if between two salts, which are mixed together in solution, decomposition should ensue, and the mixture should afterwards be found neutral, the quantity of acid, which has quitted one of the bases, must have been exactly equivalent to the saturation of the other base, also deserted by its acid. If, for example, we mingle the muriate of magnesia and sulphate of soda, the mixture continues neu¬ tral ; and hence it follows that the muriatic acid, which has quitted the magnesia, must have been exactly equal to the neutralization of the soda, deserted by the sulphuric acid. But from a calculation, founded on the proportion of the in¬ gredients of these salts, as established by Mr. Kirwan, it ap¬ pears that the soda, detached from the sulphuric acid, is not adequate to the saturation of the muriatic acid. The mix— tuie, iheiefoie, ought to be acid ; and since this is contrary to fact, we may safely infer that there is an error in the esti¬ mation of the ingredients composing these salts. No tables, indeed, can be correct, unless they stand the test of this mode of verification. Such a table has been calculated by Fischer from the experiments of Richter; but even this table seems in several respects to be of questionable accuracy. I have thought it, however, entitled to a place among the tables in the Appendix ; and I shall annex, also, a more correct one, the data of which are chiefly supplied by Dr. Wollaston’s paper on Chemical Equivalents*.
* Phil. Trans. 1814,
58
CHEMICAL AFFINITY, &C.
CHAP. II
SECTION VIII.
Experimental Illustrations of Chemical Affinity, Solution , &c.
For these experiments, a few wine glasses, or, in preference, deep ale glasses, will be required; and a Florence flask for performing the solutions.
I. Some bodies have no affinity for each other . — Oil and water, mercury and water, or powdered chalk and water, when shaken together in a vial, do not combine, the oil or water always rising to the surface, and the mercury or chalk sinking to the bottom.
II. Examples of chemical affinity , and its most simple effect , viz . solution.— Sugar or common salt disappears or dissolves in water; chalk in dilute muriatic acid*. Sugar and salt are, therefore, said to be soluble in water, and chalk in muriatic acid. The liquid in which the solid disappears is termed a solvent or menstruum. Chalk or sand, on the contrary, when mixed with water by agitation, always subsides again. Hence they are said to be insoluble.
III. Influence of mechanical division in promoting the action of chemical affinity , or in favouring solution. — Lumps of chalk or marble dissolve much more slowly in dilute muriatic acid, than equal weights of the same bodies in powder. Muriate of lime, or nitrate of ammonia, cast, after liquefaction by heat, into the shape of a solid sphere, is very slowly dissolved ; but with great rapidity when in the state of a powder or of crystals. When a lump of the Derbyshire fluate of lime is immersed in concentrated sulphuric acid, scarcely any action of the two substances on each other takes place ; but if the stone be finely pulverized, and then mingled with the acid, a violent action is manifested, by the copious escape of vapours of fluoric acid. In the common arts of life, the rasping and grinding of wood and other substances are familiar examples.
IV. Hot liquids , generally speaking , are more powerful sol¬ vents than cold ones.— To four ounce-measures of water, at the temperature of the atmosphere, add three ounces of sulphate
* I omit, purposely, the distinction between the solution and dissolution.
SECT. VIII.
CHEMICAL AFFINITY, &C.
59
of soda in powder. Only part of the salt will be dissolved, even after being agitated some time. Apply heat, and the whole of the salt will disappear. When the liquor cools, a portion of salt will separate again in a regular form or in crys¬ tals. This last appearance affords an instance of crystallization.
To this law, however, there are several exceptions ; for many salts, among which is muriate of soda, or common salt, are equally, or nearly equally, soluble in cold as in hot water. (See the table of solubility of salts in water, in the Appendix.) Hence, a hot, and saturated solution of muriate of soda does not, like the sulphate, deposit crystals on cooling. To obtain crystals of the muriate, and of other salts which observe a si¬ milar law as to solubility, it is necessary to evaporate a por¬ tion of the water ; and the salt will then be deposited, even while the liquor remains hot. In general, the more slow the cooling, or evaporation, of saline solutions, the larger and more regular are the crystals.
V. A very minute division of bodies is effected by solution.— Dissolve two grains of sulphate of iron in a quart of water, and add a few drops of this solution to a wine-glassful of water, into which a few drops of tincture of galls have been fallen. The dilute infusion of galls will speedily assume a purplish hue. This shows that every drop of the quart of water, in which the sulphate of iron was dissolved, contains a notable portion of the salt.
VI. Some bodies dissolve much more readily and copiously than others. — Thus, an ounce measure of distilled water will dissolve half its weight of sulphate of ammonia, one third its weight of sulphate of soda, one sixteenth of sulphate of potash, and only one five-hundredth its weight of sulphate of lime.
VII. Mechanical agitation facilitates solution. — -Into a wine- glassful of water, tinged blue with the infusion of litmus, let fall a small lump of solid tartaric acid. The acid, if left at rest, even during some hours, will only change to red that portion of the infusion which is in immediate contact with it. Stir the liquor, and the whole will immediately become red.
VIII. Bodies do not act on each other , unless either one or both ‘ be in a state of solution , or at least contain water. — 1. Mix some
dry tartaric acid with dry bi-carbonate of soda, and grind
2
60
CHEMICAL AFFINITY, &C.
CHAP. II.
them together in a mortar. No combination will ensue till water is added, which, acting the part of a solvent, promotes the union of the acid and alkali, as appears from a violent effervescence. It has been shown by Link*, that the water of crystallization, existing in certain salts, acts as free water in occasioning chemical action. For example, acetate of lead and sulphate of copper, both in crystals, become green when triturated together, a proof of the mutual decomposition of those two salts.
2. Spread thinly, on a piece of tinfoil, three or four inches square, some dry nitrate of copper f, and wrap it up. No ef¬ fect will follow. Unfold the tinfoil, and having sprinkled the nitrate of copper with the smallest possible quantity of water, wrap the tinfoil up again as quickly as possible, pressing down the edges closely. Considerable heat, attended with fumes, will now be excited ; and, if the experiment has been dex¬ terously managed, even light will be evolved. This shows that nitrate of copper has no action on tin, unless in a state of solution.
IX. Bodies , even when in a state of solution, do not act on each other at perceptible distances ; in other words, contiguity is es¬ sential to the action of chemical affinity. — Thus, when two fluids of different specific gravities, and which have a strong affinity for each other, are separated by a thin stratum of a third, which exerts no remarkable action on either, no combi¬ nation ensues between the uppermost and lowest stratum. Into a glass jar, or deep ale glass, pour two ounce-measures of a solution of subcarbonate of potash, containing, in that quantity, two drachms of common salt of tartar. Under this introduce, very carefully, half an ounce-measure of water, holding in solution a drachm of common salt ; and again, under both these, two ounce-measures of sulphuric acid, which has been diluted with an equal weight of water, and allowed to become cool. The introduction of a second and third li-
* Thomson’s Annals, vii. 426.
f To prepare nitrate of copper, dissolve the filings or turnings of that metal in a mixture of one part nitrous acid and three parts water; decant the liquor when it has ceased to emit fumes : and evaporate it to dryness, in a copper or earthen dish. The dry mass must be kept in a bottle.
SECT. VIII.
CHEMICAL AFFINITY, &C.
61
quid beneath the first, is best effected, by filling, with the liquid to be introduced, the dropping tube, fig. 15. pi. i. which may be done by the action of the mouth. The finger is then pressed on the upper orifice of the tube ; and the lower orifice, being brought to the bottom of the vessel containing the liquid, the finger is withdrawn, and the liquid descends from the tube, without mingling with the upper stratum. When a solution of carbonate of potash is thus separated from diluted sulphuric acid, for which it has a powerful affinity, by the intervention of a thin stratum of brine, the two fluids will remain distinct and inefficient on each other ; but, on stirring the mixture, a violent effervescence ensues, in consequence of the action of the sulphuric acid on the potash.
X. Two bodies , having no affinity for each other , unite by the intervention of a third. — Thus, the oil and water which, in Ex¬ periment I., could not, by agitation, be brought into union, unite immediately on adding a solution of caustic potash. The alkali, in this case, acts as an intermedium. The fact, indeed, admits of being explained by the supposition, that the oil and alkali form, in the first instance, a compound which is soluble in water.
XI. Saturation and neutralization illustrated. — Water,