Popular Science Monthly/Volume 17/July 1880/The New Chemistry, a Development of the Old
|←Notes on a Few of our Birds|| Popular Science Monthly Volume 17 July 1880 (1880)
The New Chemistry, a Development of the Old
By Matthew Moncrieff Pattison Muir
|Sketch of Friedrich Mohr→|
By M. M. PATTISON MUIR, F.R.S.E.
IN a former paper I endeavored shortly to summarize the more important differences between that system of chemistry which was founded on a so-called equivalent notation and the modern, or atomic phase of the science. The general conclusion to which that summary led was, that the old chemistry was empiric, while the new is scientific; but, as was there remarked, empiricism precedes science: science is the natural development of empirical statements, and is not to be regarded as entirely a new departure.
Believing, as I do, that the old and new chemistry are essentially opposed in their methods, I nevertheless am certain that the germs, at least, of many of our modern chemical theories are to be found in the statements, and even in the hypotheses, of the workers of half a century since: and in the present paper I propose to trace, in a little detail, what I believe to be a correct outline of the development of two of the more important theories of modern chemistry.
The chemical views most in vogue before the strictly modern epoch were founded more on considerations of the composition of compounds than on the actions of these compounds. Dumas introduced wider views by recalling the attention of chemists to the fact that, in order to frame even a tolerably complete system of classification, an answer must be given to the question, "What does this substance do?" no less than to the other question, "Of what is this substance composed?"
But, if we go back to the time before Lavoisier and his associates, we find that the system then predominant in chemistry was founded almost entirely on the reactions, and but to a very small extent on the composition, of chemical substances. Chemists then busied themselves continually with studying processes of chemical change; only they contented themselves with qualitative knowledge, and hence their hypotheses were for the most part extremely vague and their facts disconnected. John Joachim Beccher, born about 1630, seems to have been the first to weave together the scattered chemical facts and guesses into a consistent general theory, which was subsequently augmented and defined by Stahl (1660-1734).
Looking at the wonderful changes produced in substances by the action of chemical force, the question arose, What happens when a body undergoes chemical change? and, as burning or combustion was perhaps the commonest of all chemical changes, the question became narrowed, and chemists eagerly sought for an answer to the query, " What happens when a chemical substance burns? "
In those days natural phenomena were referred to the presence of " principles " or " essences " in the matter exhibiting the phenomena. A new principle was added to the list; and the question was sup- posed to be solved by saying that combustible substances are char- acterized by richness in phlogiston (Gr. phlogizo = burn, or inflame), and that when they burn they lose this principle, so that the burned substance, or calx, consists of the original substance minus phlogis- ton.
The phlogisteans seem to have regarded their hypothetical prin- ciple as a modified form of fire, as fire confined in a material substance; but, as they gave no definition of fire, beyond saying it was one of the four elements, it was scarcely to be expected that they should define phlogiston. By restoring phlogiston to the burned substance, said the theory, the original matter is regenerated. Some substances, e. g., charcoal, are especially rich in phlogiston, and metallic calces may be converted into metals, i. e. , may be unburned, by heating them with charcoal. Thus the phlogisteans regarded the phenomena which they studied in a purely qualitative manner: they asked only, What does this or that substance do under given conditions? not being aware that a full answer to this question can only be given when the other question How much of some given effect is produced by a given quantity of this body under stated conditions? had been an- swered.
The introduction and use of the balance carried the day in favor of those who opposed phlogistic views. If a substance loses some- thing when it burns, it must weigh less than before burning as a fact it weighs more therefore it has not lost but gained something.
" Nay," replied the phlogistean, " it has lost something, but the weight of this something can only be expressed by a negative quantity."
" But a something with such properties is an absurdity," replied the opponent; " therefore it has no existence, and therefore your theory is utterly false."
The anti-phlogistean triumphed, and the principle of levity was banished from chemical science. But the principle returned in a modified form. Lavoisier, who opposed the Beccheriah theory of phlogiston with signal success, himself propounded a theory of the constitution of solids, liquids, and gases, in which the " subtle prin- ciple " " caloric " played an important part. Lavoisier regarded oxy- gen as what he termed " concrete oxygen " plus a something caloric; indeed, he appears to have looked on all substances in the concrete state as solids, and to have supposed that the addition of a certain quantity of caloric to these caused them to become liquids, while the addition of a further quantity of caloric produced gases. Thus chemists seemed obliged to imagine a something in addition to the gross or ponderable matter of which bodies are composed, in order to account for the properties of these bodies. As Science has advanced she has been able to define what this something is; at least, she has defined it more clearly than the older workers could do.
I havesaid, as Science has advanced she has defined the unknown something; but it should be remembered that that wonderful book, which contains according to the greatest authorities the germs of all our modern advances, was written sixty years before Lavoisier's time. Sixty years before the apparent overthrow of the theory of phlogiston, Newton had laid the foundations of the science which was to reveal the true lineaments of that Unknown whom the phlogisteans ignorantly worshiped.
We have learned to extend the meaning of the word thing we speak of " the power of doing work " as a measurable and definite thing although not as matter: and we know that when a body burns it loses a certain amount of this power of doing work, or, as it is more shortly put, of energy. As usual, it is a question of words. The older workers could not define phlogiston; we are able to define energy, and therefore we can see clearly where they saw but darkly. Chem- istry now acknowledges that the properties of a compound are not only determined by the composition of the matter of that compound, but by the amount of energy associated with that collocation of matter. She has been able to point out many instances of compounds composed of the same matter, but possessed of different amounts of energy, and, at the same time, of very different properties. And, moreover, chem- istry aided by physics has concluded that the properties of a body " are dependent on the variations of the energy of the body, and not on its total value," and therefore that " it is unnecessary, even if it were possible, to form any estimate of the energy of the body in its stand- ard state." (I quote from that remarkable little book of the late Professor Clerk Maxwell, " Matter and Motion.")
Whenever Science made the advance from the vague conception of " principles " and " imponderable matter " to the definite conception of "mass," "motion," and "energy," she was able to recognize the truth which lurked under the cumbersome and inexact nomenclature of the phlogistean chemists.
I have said that, as usual, the dispute between the phlogisteans and their opponents was proved to be a question of meaning of words: as usual, also, subsequent research showed that, while both were wrong, both also were right.
Composition is important, but composition is not all. The burned body has properties differing from those of the unburned body, because it has lost a certain amount of " the power of doing work "; but it has a less power of doing work because it is possessed of a structure differ- ent from that which it possessed before. Composition and properties, energy and structure, are closely connected: to determine the exact relations existing between these, under stated conditions, is still the fundamental problem of chemical science.
We can define energy: the phlogisteans could not define phlo- giston. But in the ethereal philosophy of the future will it not be said of the present workers in science that they could not define ether, but even spoke of it at times as " not gross nor ponderable matter "? The theory of phlogiston was continued and developed in the theory of caloric: the theory of caloric is vastly extended, simplified, and ren- dered definite in the theory of energy, and the theory of energy seems destined to be largely extended by the ethereal theory now in its infancy.
Mankind has until lately been content with space of three dimen- sions, but the bolder and more dashing spirits among the mathema- ticians have dared to look forward to a better world than this where they may revel in space of four dimensions. What a strange world must that be! what a fearful place for a mathematical examination, when we remember that the inhabitants thereof if there be inhabi- tants may turn spherical hollow balls inside out without tearing or breaking them!
While we look forward to the future of science with hope, I think we ought not to look back on the former workers without respect.
But I must pass on to consider the second of the great theories which have paved the way for the doctrines of modern chemistry. The germ of the modern ideas of substitution, valency, atom-linking, etc., is, I believe, to be found in the pure dualism of Berzelius; and, moreover, the influence of the dualistic ideas of that great chemist seems to me easily traceable in the essentially unitary system of modern chem- istry. The chemistry of Lavoisier centered around the wonderful sub- stance whose properties he so carefully studied. The teaching of the great founder of modern chemistry was saturated with ideas suggested by the study of oxygen. The compounds of oxygen were divided by Lavoisier into two groups, bases and acids: when these reacted chem- ically, a salt that is, a body made up of base and acid was produced. Berzelius developed these ideas until he had constructed a complete and beautiful theory, viewed in the light of which all compounds were of analogous structure. Every chemical substance was made up, ac- cording to the Swedish chemist, of two parts; these parts might them- selves be composed of simpler parts, or they might be truly element- ary. The two parts of a compound were respectively endowed with .positive and negative electricity. When two bodies combined, the positive electricity in one neutralized the negative electricity in the other; hence the phenomena of light and heat noticed in chemical combination. An element might contain an absolutely greater quan- tity of positive electricity than another and nevertheless belong to the electro-negative series of elements: thus sulphur and oxygen readily combine to form a substance which, when dissolved in water, yields an acid. But oxygen and sulphur are both electro-negative elements. Berzelius supposed that sulphur contained a large quantity of both electricities, the negative predominating. When this element com- bined with oxygen, the positive electricity of the sulphur was Sup- posed to be neutralized by the negative electricity of the oxygen, so that the negative electricity of the sulphur was concentrated or ren- dered more apparent. The affinity between oxygen and silver is less than that between sulphur and oxygen, because, said Berzelius, silver contains mainly positive electricity, but a smaller quantity than is found in sulphur. The product of the union of oxygen and sulphur, i. e., of oxygen with an electro-negative body, belongs to the class of acid oxides; the product of the union of oxygen and silver, i. e., of oxygen with an electro-positive element, belongs to the class of basic oxides.
If this view of the composition of oxides were granted and a most ingenious and plausible theory it was why should we not proceed a step further and say that an acid acts so readily upon a base, because, in the first, negative electricity predominates, while the prevailing- electricity in the latter compound is positive?
And, in further support of this view, could it not be experimentally demonstrated that when a salt, such as sulphate of sodium, is decom- posed by the electric current, the soda goes to the negative pole, while the sulphuric acid appears at the positive pole? The experiment of decomposing a solution of sulphate of sodium was frequently per- formed, and the fact that, if the solution were colored with litmus, that portion around the negative pole retained its blue color, while that around the positive pole became red, was regarded as conclusive evidence of the dualistic structure of the salt operated upon.
But, about the year 1834, Dumas told the chemical world that chlorine was capable of " laying hold of the hydrogen of certain bodies and replacing it atom for atom." If this be so, said Berzelius, the compound formed must differ essentially from that from which it is derived. Chlorine is an electro-negative element, and, if it enter into a compound in place of the electro-positive hydrogen, the original compound and the new compound can present no points of analogy. The theory seemed correct, but unfortunately the chlorinated body did present very marked analogies with that from which it had been pro- duced. Berzelius attempted many exj)lanations, invented many new compound groups of atoms, which should be supposed to enter into the composition of the new bodies discovered by Dumas; but his electro-chemical theory was doomed. It was gradually abandoned by most chemists, and the substitutionists carried the day.
Berzelius had largely availed himself of certain facts, which showed that, in series of reactions, it was sometimes possible for a group of dissimilar atoms to remain intact, to move about, so to speak, from one compound to another without falling to pieces. Reasoning on these facts, he constructed formulae for all compounds, which formulae were made up of two parts, or radicles. The idea of compound radi- cles was thus closely associated with the dualistic theories of the Ber- zelian school. The new school, led by Dumas, finding dualism insuffi- cient to explain many weighty facts, naturally waged war against the fundamental conception of compound radicles, but they were soon obliged to accept the essential truth of the theory which they at first opposed. Liebig and Wohler's research on oil of bitter almonds led to the discovery of a number of compounds, exhibiting many general analogies, which could best be explained by supposing the existence in each of a compound radicle, or group of atoms. When it became necessary once more to adopt the idea of compound radicles, the theory of substitution was found to be strengthened, not weakened, thereby. Many reactions were made clear by supposing that an element might be substituted by a group of elementary atoms, by a compound radicle. But in adopting the idea of compound radicles the substitutionist yet maintained tbat the chemical compound was a distinct whole, made up of parts he admitted, but, nevertheless, having these parts so modified and merged in one another that the resultant acted as an homogeneous compound. Thus, when the new school likened the ethers to the me- tallic oxides, they did not mean to assert that the molecule of ether was composed of two parts, ethyl and oxygen, held together by elec- tric bonds, and ready to part company without difficulty; nor, in as- serting that ether was one substance, and not a dualistic system, did they deny the existence of a structure within the molecule of ether. They admitted the existence of a closer relationship between the atoms of carbon and hydrogen constituting the group ethyl than between these atoms and those of oxygen, and they generalized the reactions and analogies of ether, by saying that it might be regarded as sodium oxide in which both sodium-atoms had been substituted by two com- pound atoms of ethyl. Berzelius had himself likened the ethers to oxide of potassium, and by doing this the great apostle of dualism had paved the way for the advance of the unitary theory.
That portion of the dualistic doctrine which was embodied in the theory of compound radicles was adopted by the unitary schools, but adopted in a modified form; the effects of this modification were not long in making themselves felt.
Berzelius, in his later works, had been ready to give a dualistic formula to any compound without stopping to inquire into the facts known about that compound; he had tended to forsake the only true scientific method, and to substitute the vagaries of his fancy for the facts of nature. The new school averred tbat " compound radicle " was an expression generalizing a class of facts; that the reactions of bodies were most simply explained by supposing tbat when acted on by chemical force the little parts of these bodies behaved as having a definite structure; and that therefore the formula of a given body might be written as containing different compound radicles under dif- ferent conditions.
The fault of the old chemistry was that more attention was paid to symmetrical formulae than to reactions; the merit of the new con- sisted in bringing the student once more back to nature.
And the appeal to nature was answered, and answered abundantly. The new conception of compound radicles was rich in results; from it there was developed first, the theory of types, and, subsequently, the wider theory of valency, which has led to that of atom-linking, and these in their turn have reacted on the older and more fundamental notions of the science, and have given a new meaning to such terms as " chemical " and " mechanical actions," " compounds " and "mixtures," etc., while, at the same time, they point the way to the chemistry of the future when we shall have gained a definite conception of the inner mechanism of the molecule, and of the laws which regulate the combinations of molecules in groups, and the decompositions of mole- cules with subsequent formations of new atomic systems.
Let us shortly examine these ideas. If sodium be thrown on to water, caustic soda is produced, a substance made up of hydrogen, oxygen, and the simple radicle sodium; by another reaction a sub- stance can be obtained consisting of hydrogen, oxygen, and the com- pound radicle nitryl (NOJ. These two bodies have analogous for- mulas, Na OH and (N0 2 ) OH, they may both be regarded as derived from water, H HO, by the replacement of otfe half of the hydrogen by a radicle; in one case by Na, in the other by N0 2 . Again, the whole of the hydrogen in water may be replaced by sodium, with production of the compound sodium oxide, Na 2 0.; but in many of its reactions this compound is the analogue of common ether, which is also a com- pound of oxygen with a (compound) radicle ethyl, and has the for- mula (C 2 H 5 ) 2 0. Now, these substances, Na OH, (NO) OH, Na 3 0, and (C a H 5 ) 2 0, both on account of the methods by which they are produced, and because of their general reactions, may be classed to- gether as derivatives of water, or may be said to belong to the water- type. Similarly, other types have been instituted, and large groups of compounds have been brought into the same class as being all refer- able to one parent type. This step in advance is evidently an out- come of the theory of compound radicles; without that conception a system of classification by types would have been impossible.
But it was found that while such compound radicles as C 2 H B or N0 2 were capable of replacing but one part by weight of hydrogen in water, other compound radicles, such as CO or C 2 H 4 , were capable of taking the place of two parts by weight of hydrogen. Comparing to- gether these two sets of radicles, it might be said that CO = 2 N0 2 or C a H 4 = 2 C 2 H 6 , so far as the power of combining with hydrogen was concerned. This conception of binding power being extended to the elements, and being deepened and widened by laborious experimental researches, led to the general theory of valency, which included in itself the essential features of the older doctrine of equivalents.
Having thus gained the conception of a definite binding power as applicable to elementary atoms or groups of atoms, it followed, as an almost necessary deduction, that the smallest parts of chemical com- pounds which existed as distinct chemical entities, i. e., the molecules, must have a definite structure: that the parts (atoms) of the little sys- tems must be arranged in accordance with the valencies, or binding powers, of these parts.
Hence, given the number of atoms in a molecule, and the valency of each atom, it became possible to calculate the number of different arrangements of these atoms which could be produced; and careful experiment has often succeeded in preparing all the different, theoreti- cally possible, compounds. The difference of properties of such com- pounds, i. e., of compounds the molecules of which are constituted of the same number of the same atoms, but differently arranged, is attempted to be indicated in the " structural " or " rational " formula? of modern chemistry.
Berzelius spoke of compounds composed of parts held together by mysterious bonds: the idea survives in these structural formulae of to- day, only we are now able to define what we mean by the smallest part of a compound having a chemical existence, and we have gained certain generalizations which enable us to trace with some degree of accuracy the relationships which exist between the inner parts of these smallest chemical wholes. We appear to be now fairly embarked in the prosecution of molecular dissection, and our chief guide is the the- ory of valency, itself a development of the dualistic chemistry.
Each elementary atom, I have said, seems to have the power of directly binding to itself a maximum number of other atoms; but it would further appear as if the groups of atoms thus produced had also a certain binding power, but this more indefinite than the atomic bind- ing power, and very variable under different physical conditions. This atomic binding power appears to have a fixed maximum value, but not always to reach the maximum. What is the exact way in which the binding power or valency of the elementary atoms is influenced by definite changes in physical conditions? This is one of the most im- portant unsolved problems of general chemistry.
Then, again, granting the existence of an inner structure to the molecule, granting that groups of atoms do exist in the molecular building, does the fact, that in a certain reaction certain atoms are withdrawn as a group, prove that these existed in the form of the same group in the original molecule? In other words, do our struc- tural formulae express the relative collocation of atoms within the molecule while the molecule is unacted on by extraneous force, or do they merely roughly represent the condition of things when the mole- cule is in a state of strain, because of the stress between its parts and those of another molecule, or molecules, brought within its sphere of action? Here is another question which can only be answered after much experimental evidence has been accumulated. Now, these ques- tions, I make bold to say, are the direct outcome of the dualism of Berzelius, modified by the unitary chemistry of Dumas and his fol- lowers.
If we glance back on the development of the two theories, the course of which I have endeavored to outline, we find that both began with a purely qualitative study of reactions, but that it was only when to this had been added the careful use of weights and measures that any solid advance became possible. Further, we find that the older theory was founded chiefly on a study of reactions, while that which was broached after the time of Lavoisier was founded most largely on a study of composition. With the phlogisteans function was of para- mount importance; with the dualists composition was all. The mod- ern theories, which have been developed from these, have attempted, with varying success, to combine both considerations. And if we examine the latest advances of theoretical chemistry we still find it at work on these two lines of advance. The composition of chemical compounds is studied by the majority of chemists; but the general laws of action of chemical force itself have of late received most im- portant elucidation.
Again, if we look to the " lines of advance along which dynamical science is working its way to undermine, at least, the outworks of chemistry," we can distinguish two, essentially the same, lines as were used by the two classes, whose theories I have dealt with in this paper. " One is conducted by the help of the hypothesis that bodies consist of molecules in motion, and it seeks to determine the structure of the molecules and the nature of their motion from the phenomena of por- tions of matter of sensible size. The other line of advance, that of thermo-dynamics, makes no hypothesis about the ultimate structure of bodies, but deduces relations among observed phenomena by means of two general principles, the conservation of energy and its tendency toward diffusion." (Clerk Maxwell, " South Kensington Science Con- ferences," 1876, p. 145.)
I have thus sought to substantiate the claim of the new chemistry to be a development of the old. I believe that, if this claim is granted, the conclusion to be drawn must be, not that the old is better, but that to return to that which is admittedly an early stage of development would be to misread all the teachings even of the old chemistry itself.
In examining the progress of Science we see that she is not afraid to retrace her steps, and that she is able to retain and develop all that is probably true, while rejecting all that is proved to be false; and, when we learn that she does this, can we hesitate to find in her history the " promise and potency " of a mighty future? Popular Science Review .
vol. xvii. 26
- "Popular Science Review," January, 1878.
- In the paper referred to, I briefly sketched the history of the development of the older doctrine of "Equivalents" into the modern hypothesis of "Valency."