verilication^ P^'^"^ ^^^ establishing the doctrine of definite proportions. I do not refer chiefly to his ti-ansformation of the atomic theory into that of chemical equivalents, though it has a more positive character, and tends to restrain the student from wandering after inac- cessible objects, to which the first might tempt him, if not judiciously directed. The substitution would be of high value, lao doubt, if it were not less a change of conception than an artifice of language. Nor have I in view the ingenious expedients by which Wollaston popularized numerical chemistry by rendering its use more clear and convenient. A greater service, in our present view, was his furnishing us with the indispensable complement of E/ichter's discoveiy, by establishing the theory in regard to the acid salts, since extended by analogy to the alkaline THE APPLICATIONS OF NUMERICAL CHEMISTRY. 355 salts. The case of the acid salts was j^erhaps the most unfavourable possible for the ascertainment of the prin- ciple of invariable proportions. Wollaston effected the proof in the most satisfactory manner; and this sjjecial confirmation of the principle is considered, from its nature, the most decisive of all.
Such has been the logical and historical progress of the researches which have constituted numerical chemistry as it is now. We can represent by an invariable number, appropriated to each of the different elementary bodies, their fundamental relations of chemical equivalence, whence, by very simple formulas, immediately expressing the laws just indicated, we easily pass to the numerical composition proper to each combination. No further evidence of the truth of the doctrine is needed, than the fact of so many illustrious inquirers having attained the same view by ways which each one opened for himself, and all agreeing as to its positive application to all cases of importance, differing only as to the mode of expression of the results, in as far as the atomic theory left it indeterminate, and therefore optional. But we must glance at the difficulties thrown in the way of its application by a consideration of the aggregate of chemical jihenomena, in order to form a clear idea of the final improvement of which this doctrine yet stands in need.
Among the points which are beyond dis- Scope of appli- j)ute, it is, first, evident, and no chemist has cation of Nu- ever doubted it, that substances differ as merical Che- much in the proportion as in the nature of ""^^ry. their constituent principles. It is an axiom of chemical philosophy that an}^ change whatever in the numerical composition causes a change in the whole of the specific properties, in a more marked degree as the alteration is greater. Varied and gradual above all others as are the proportions produced by the chemical phenomena proper to living bodies, they afford a striking confirmation to this universal maxim. Therefore, in the lowest stages of chemical analysis, chemists have always endeavoured to assign, as a characteristic property, the proportion of the elements of each substance, as far as was possible: and when this was omitted, it was on the understanding that 356 POSITIVE PHILOSOPHY.
the proposed combination admitted of only a certain pro- portion; as in the case of the neutral salts.
Again, it has long been acknowledged that there always exists, between any two substances, a certain minimum and maximum of recijiroeal saturation, beyond or short of which all combination becomes impossible. At the utmost, certain variations, themselves restricted, have been sup- posed procurable. Berthollet established, more directly than any one else, the general and necessary existence of these limits of combination, — one of the principal cha- racters which distinguish it from simple mixture. It is clear that the two extreme degrees of all combination must be subject to special and invariable proportions: and, as all agree in this, all argument about the opposite doctrines of indefinite and definite proportions is reduced to the question whether the passage from the minimum to the maximum of saturation can be effected gradually and almost imperceptibly, or whether it takes place always abruptly, through a small number of well-marked degrees.
Thirdly, the possibility and actual existence of interme- diary definite proportions ai'e admitted by all chemists, who can have no other dispute than about the greater or smaller generality of such a property. We have seen that the idea of neutrality must, sooner or later, bring after it that of a determinate and unchangeable proportion; and the gradual development of chemical knowledge has ex- tended this character to more and more varied cases. Berthollet disclosed several other causes of definite pro- portions, which were entirely misconceived before his time, and which may meet in almost all combinations, modifying certain circumstances of the phenomenon. The precise question now is, therefore, whether, besides these deter- minate compounds, subject to fixed proportions, within the two limits of possible combination, there does or does not exist, in general, a continuous series of other intermediate compounds of a less marked chai'acter; in a word, whether definite proportion constitutes the rule, as is now generally supposed, or, as Berthollet endeavoured to establish, the exception. This is now the only dispute. It is no deroga- tion from the interest of the doctrine of definite proportions to say, as some preceding considerations compel us to do.
VALUE OF THE THEORY, 357 that the decision of this disputed point is not of the importance commonly supposed. The doctrine has tended to simj)lify the genei'al problem of chemistry; but it must not be supposed that the solution would have been impos- sible without this aid: — it would have been simply more difficult and less precise. The eminent chemists who con- curred in establishing the doctrine were naturally engrossed by that labour; but their successors, who find numerical chemistry constituted to their hand, must beware of losing sight in it of the true scientific aim of chemistry. They must not linger in this vestibule of the science, to the neglect of the direct construction of Chemistry itself, — an enterprise scarcely begun, and to which it is high time that attention should be once more fully directed.
If we inquire, as we must do, how far the doctrine of definite jDi-oportions is irrevocably established, we shall bear in mind that the founders of numerical chemistry have accomjjlished that chief part which depends on an investigation of all known compounds, leaving only the question whether the doctrine is compatible with certain chemical phenomena, neglected during its formation, and remaining to be since referred to it.
The first general objection relates to the.
important phenomenon of dissolution, evi- dissolution^ dently 2:)ossible in an infinity of different pro- portions. It must be actnowledged that the distinctions between the state of dissolution and that of combination, by which the difficulty has been met, afford little satisfac- tion. In my opinion the only eifectual reply must consist in the extension of the principle of definite proi:)ortions to the phenomena of dissolution; and, difficult as it may be to do it, it does not seem to me impossible. The way is by the use of an hypothesis already proposed for other cases in which it miglit appear less admissible. All the successive degrees of concentration of the liquid must be regarded as simple mixtures of the small number of definite dissolutions which shall have been established, either between themselves or with the dissolvent, in the manner of habitual mixtures of water with alcohol, with svilphuric acid, etc. In any case, the positive verification of this hypothesis must be extremely delicate. Furthermore, to 358 POSITIVE PHILOSOPHY.
render the study of dissolutions fully rational, in this point of view, it is necessary to combine with it that of other analogous chemical phenojnena, relating to the absorption of gases by liquids or by porous solids. All these different modes of molecular union are often energetic enough to resist influences able to destroy certain combina- tions, properly so called: why should they not be, like them, subject to the rule of definite proportions, if that rule is truly a fundamental law of nature?
The next case, that of various metallic allovs alloys, is very extensive, though more particular. The difficulty lies in the question whether these are cases of combination or of mixture. The state of combination has been taken for gi'anted in the case of alloys; whereas the general application of the principle of numerical chemistry reqidres that they should be mixtures; while, again, it is diflicult to conceive of such a mixture of solids as could resist perturbing influences which would appear to be necessarily destructive; as great changes of temperature, the influence of crystallization, etc. The question can be decided only by a series of special experiments, devised to find the general limits of the permanence of unquestionable mixtures; and the results might be extended to other questions of numerical chemistry, as of certain oxides, on which explanations have been hazarded too freely. When a true chemical theory of mixtures is established on a proper l)asis of experiment, and we leave off referring to an hypothesis of mixture all cases in which combination seems susceptible of an inde- terminate proportion, in order to bring them under the law of definite propoi'tions, we shall get rid of a formidable objection to the principle of numerical chemistry.
The remaining case constitutes the greatest Hubstances obstacle of all in the way of the generalization of the law of definite proportions: and if it cannot be surmounted, the law sinks to the rank of an empirical rule, fit for nothing more than facilitating a certain order of chemical analyses. I refer to the class, anomalous in this view, of substances called organic. And this is the effect, in fact, of the declaration of the chemists of our time, that organic substances do not come under THK PRIXCIPLK OF DUALISM. 359 the principle of definite proportions. It amounts to saying that the law rules all the elements, except oxygen, hydrogen, carbon, and azote. The division between inorganic and organic chemistry is merely scholastic; for ail chemistry is, by its nature, homogeneous, — -that is, inorganic. And thus, if we admit of the enormous exception of the nume- rical composition of so-called organic substances, the doctrine of definite proportions is overthrown as a rational theory. As it evidently cannot be founded on any a priori considerations, it is only by a strict generality that it can become a rational theory.
If we could not hold at once the grand Application of principle of the dualism which pervades the principle chemistry, and constitutes its homogeneous °^ (uiahsm. character, and the doctrine of definite proportions, I should not hesitate to sacrifice the latter: for it is more important for chemical jirogress to gi-asp the great principle of syste- matic dualism, than to advance our investigations by the use of the numerical rule. But there is not, in fact, any incompatibility between these two means of progress: and such a brief sketch of my conception on this subject as my limits allow may show how the doctrine of definite pro})or- tions can be duly generalized only by discarding organic chemistry as a separate body of doctrine, and extending the principle of dualism to all organic compounds.
If we are to include all organic compounds under one uniform system of chemistry, properly so called, we must refer to physiology, vegetable and animal, the study of the numerous secondary substances which owe their transient and variable existence to the development of vital phenomena, and which have no scientific interest except under the head of biology. We shall see, under that head, hereafter, what the precise classification is; and all that we have to do with it now as to show that it proceeds from the fundamental distinction between the state of death and that of life. The second, and most extended class of oi'ganic substances is chiefly composed of mixtures which, as such, admit of all imaginable proportions, within the limits of vital conditions. As for the substances which, exhibit real combinations, we must conceive of them as subject to the law of definite proportions; but the com- 360 POSITIVE PHILOSOPHY.
plexity, and yet more the instability of sucli compounds will probably for ever forbid their being successfully studied under the numerical jjoint of view, which is indeed of very inferior interest in biology. — Even after this clear- ing of the field, we could not accomplish the desired generalization if we had not taken a new stand with regard to the ternary and quaternary substances contemplated by ordinaiy chemistry. The rigorous dualism which I have before, and in a higher view, shown to be necessary, seems to supjily, naturally and finally, the needs of the doctrine of definite proportions.
As long as chemists persist in regarding organic com- binations as ternary and quaternary, — that is, in con- founding their elementary with an immediate analysis; — while oxygen, hydrogen, carbon and azote are regarded as immediately united, the compounds from them which must be recognized as distinct, after the severest sifting, *will be enough to constitute an invincible objection to the princij^le of a numerical chemistry. But if they become binary compounds of the second, or at most the third order, whose principles are formed by the direct and binary combination of those three or four elements, we find our- selves able to represent all the actual numerical varieties established by an elementary analysis, conceiving, for each degree of combination, a very small number of distinct and entirely definite proportions. — In the ternary case, — appro- priate to compounds of a vegetable origin, — their three elements may be united in three kinds of binary combina- tions. Combining these again, still employing at once oxygen, hydrogen, and carbon, we have three principal classes of comj^ounds of the second order. But then again, each term of the new compounds really corresponds to two distinct substances: and thus, while admitting only one proportion for the binary composition of tliese bodies, we have already provided for the numerical composition of twelve substances at present called ternary. But, further, we are compelled to suppose at least three different pro- portions for each l)inary combination: one producing per- fect neutralization, and the other two the extreme limits of the reciprocal saturation: and chemical analogies indi- cate a much larger number of comi^ounds. Putting those APPLICATION OF THE PRINCIPLE. 361 aside, "we have thirty-six compounds, without goiug beyond the second order, by the known combination of three elements on the principle of dualism. We are also entitled now to conceive of a third possible combination between oxygen and carbon, or between carbon and hydrogen, etc., which already furnish two, after being long supposed to admit of only one. Hence, and in view of all these con- siderations, we may be assured that by dualism we might completely and naturally subject to the law of definite proportions eighty-one compounds of the second order, formed from oxygen, hydrogen, and carbon; and this would unquestiouably more than suffice to represent the elementary analysis of all distinct substances in the range of vegetable chemistry.
Passing on to the quaternary case, — characterizing what is called Animal Chemistry, — it seems as if the principal class of compounds of the second order must be more numerous than in the tei-nary case: but the indispensable condition of employing all the four elements at once restricts the classes to three. But when we examine the terms of the secondary compounds, we find that while two represent only one compound each, a third represents five. Thus, the three pairs of compounds yield fourteen of one proportion, and forty-two of the three proportions indi- cated in the last case. But applying, at each degree, the rational rule of a triple binary combination, without stop- ping at the inevitable gaps of our existing chemistry, we find ourselves in possession of ninety-nine compounds of the second order, now regarded as quaternary. This is probably a larger number than a rational analysis of animal substances will be found to require. Moreover, as animal substances have undergone a greater degree of vital elaboration than vegetable matters, it would be philo- sophical to admit, with respect to them, the possibility of a higher order of composition, such as physiological com- binations must eminently tend to realize. On such an hypothesis, without going beyond the third order, we might obtain ten thousand perfectly distinct compounds from these four elements, all formed by an invariable dualism, and strictly subject to the law of definite propor- tions. It is true, nature would not jjermit the realization 362 - POSITIVE PHILOSOPHY.
of more than a small part of these speculative combina- tions; but I have pursued the consequences of my con- ception to this extreme ideal limit, to show how abundant are the rational resources supjjlied by this new theory for the generalization of the laws of numerical chemistry. If this view is not followed up, or some equivalent one pro- posed, it is evident that we must give up the doctrine of definite proportions as a law of natural philosophy, and return to Berthollet's theory, merely enlarging the cases of fixed proportions which he admitted. In the present state of the question there is no other choice. But my theory having been, not instituted for this destination, but natu- rally arrived at by another way, and with higher views, and proceeding from established principles, to meet the needs of chemical philosophy, seems to me to be presump- tively entitled to a future, and pei'haps speedy realization. This account of the present aspects of the doctrine of definite proportions will enable any one to judge of its real progress from its institution to this day; of the conditions which must be fulfilled before its principle can be con- verted into a great law of nature; and of the rational course which alone can lead to such a final constitution of numerical chemistry.
363 CHAPTER IV.
THE ELECTRO-CHEMICAL THEORY.
FROM the beginning of modern cliemistry, Relation of the chemical influence of electricity Electricity to manifested itself uneqiiivocally in many im- Chemistry, portant phenomena, and above all, in the grand experi- ment of the recomposition of water by the direct combina- tion of oxygen with hydrogen, effected by the aid of the electric spark. But the special attention of chemists was not strongly drawn towards this agency till Volta's im- mortal discovery disclosed its princij^al energy, in render- ing the electric action at once more complete, more pro- found, and more continuous. Since that time, various series of general phenomena have taught us that electi-icity is a chemical agent more universal and irresistible than heat itself, both for decomposition and combination. The danger now is of exaggerating the relation it bears to the general system of chemical science. Though chemistry is united to j^hysics by this agency more than by any other, it must yet be remembered that the two sciences are distinct, and that there should be no confounding of chemical with electrical properties. In order to ascertain with precision what are the relations of chemistry with electrology, we must briefly review the gradation of ideas which have led up to the present electro-chemical theory, as systematized by Berzelius.
The first important chemical effect of the History of voltaic influence was the decomposition of tlie case. water, established by Nicholson in 1801. It Nicholson's was a necessary result of examination into discovery, the action of the pile, without any chemical intention. It confirmed a truth before well known: but it had a high chemical value, as revealing the chemical energy of the instrument; and it thus constituted the starting-point of 364 POSITIVE PHILOSOPHY.
eleetro-clieinical research. We may even refer to this origin the first attempts at founding a general theory of electro-chemical phenomena; for the conception offered hy Grothuss, to explain Nicholson's observation by the electric polarity of molecules, contains the germ of all the essential ideas which have expanded, according to the requisitions of the phenomena, into the present electro-chemical theory.
The analytical power of the voltaic pile having been once discovered, it was natural for the chemists to apply the new agent to the decomposition of substances which had hitherto resisted all known means. The first series of , attempts produced, after a few years, the coverv ^ ^'^ brilliant discovery by the illustrious Davy, — of the analysis of the alkalies, properly so called, and the earths. Lavoisier's theory, which showed that every salifiable base must be a result of the combina- tion of oxygen with some metal, had foreshown this analysis; but no chemical means had sufficed to effect it. It was believed in, in spite of Berthollet's discovery of the true composition of ammonia; and the brilliant result obtained by Davy was an easy consequence of a discovery completely jirepared for. M. G-ay-Lussac soon followed, with a more difficult but less striking achievement, — the confirmation, by a purely chemical process, of the electrical analysis of potash.
Nicholson's observation having originated electro- chemistry, and Davy's given it a great impulse, the next step was to investigate the chemical influence of elec- tricity, in a scientific view. This was effectually, though indirectly, determined by Davy's great feat; for by it chemistry was proved to have achieved the most im- portant, and hitherto inaccessible analyses: and in fact, the science has not since made any essential acquisition. Electro-chemical action was presently and permanently subjected to direct and regular study; and it was irrevoc- ably constituted a fundamental part of chemical science , when Berzelius accomplished his series of extension investigations on the voltaic decomposition of all the salts, and then of the principal oxides and acids. It was in consequence of these researches that the habitual consideration of electric i:)roperties has SYXTIIESIS. 365 assumed a growing importance in the chemical study of all substances, which are now scientifically divided into the classes of electro-negatives and electro-positives. It was thus the privilege of Berzelius to be the first to con- ceive of the electro-chemical theory under an entirely systematic form.
One condition remained to be fulfilled, to s - tl f • 1 give its due scientific character to this new process, branch of chemistry. The voltaic action had thus far been regarded only analytically: — it must be re- garded synthetically also. This was done by the labours of Becquerel, who fully established the synthetical in- fluence of electricity, fitly applied; and who, moreover, employed it in effecting new and valuable combinations, hitherto impracticable. From this procedure arose the necessity of modifying the method of experimentation. The apparatus which was powerful enough to decompose was much too powerful to combine, because it would pro- bably decompose the immediate principles which were in- tended to be combined; and thence arose the method of employing the protracted action of very feeble electric powers, — every advantage being given to the disposition of the substances to be acted upon. M. Becquerel did this very successfully by operating with a single voltaic element, and seizing each substance in the T.^„„„Q,.ar ■ ' '^ -.,., l>ecquerel s.
state called nascent, which is agreed upon as most favourable to combination. This change of pro- cedure is the distinctive honour of M. Becquerel. He not only determined direct combinations, not before obtainable, but exhibited in others, which were obtainable, the re- markable property of clearly manifesting their geometrical structure, through the slowness and regularity of their gradual formation; — a character especially marked in the case of certain metallic sulphurets, some oxides, and several salts. It does not lie within our province to point out the results of this method in regard to the natural history of the globe, in explaining a great number of mineral origins, when the time for such concrete questions shall have arrived. It is more in our way to observe the importance of these labours in bringing up chemical syn- thesis to something like harmony with the progress of 366 POSITIVE PHILOSOPHY.
analysis, — favoured as the latter pursuit had been by the ease with which we destroy, in comparison with the diffi- culty with which we recreate. Once more, these researches of M. Becquerel have comj^leted the general constitution of electro-chemistry, which, being henceforth at once syn- thetical and analytical, can only expand in one of these two directions, however great the improvements remaining to be attained.
Such has been the filiation of electro -chemical discoveries, since the beginning of our century. A little attention to the great phenomenon which was the original subject of the electro-chemical theory will show how this study has gradually led to a new fundamental concejition for the whole of Chemistry.