|The opening section to William Whewell's 1840 “The Establishment and Development of the Idea of Chemical Affinity”, in which he cogently covers the history of affinity chemistry, from 1718 to 1830s. |
The following is the full text of Book 4, Chapter II, wherein Whewell covers twelve main points on affinity; partially-hyperlinked with added (related) images and brackets [see alsos]:
1. The earlier chemists did not commonly involve themselves in the confusion into which the mechanical philosophers ran, of comparing chemical to mechanical forces. Their attention was engaged, and their ideas were molded, by their own pursuits. They saw that the connection of elements and compounds with which they had to deal, was a peculiar relation which must be studied directly; and which must be understood, if understood at all, in itself, and not by comparison with a different class of relations. At different periods of the progress of chemistry, the conception of this relation, still vague and obscure, was expressed in various manners; and at last this conception was clothed in tolerably consistent phraseology, and the principles which it involved were, by the united force of thought and experiment, brought into view.
2. The power by which the elements of bodies combine chemically, being, as we have seen, a peculiar agency, different from mere mechanical connection or attraction, it is desirable to have it designated by a distinct and peculiar name; and the term affinity has been employed for that purpose by most modern chemists. The word “affinity” in common language means, sometimes resemblance, and sometimes relationship and ties of family. It is from the latter sense that the metaphor is borrowed when we speak of chemical affinity. By the employment of this term we do not indicate resemblance, but disposition to unite. Using the word in a common unscientific manner, we might say that chlorine, bromine, and iodine have a great natural affinity with each other, for there are considerable resemblances and analogies among them; but these bodies have very little chemical affinity for each other. The use of the word in the former sense, of resemblance, can be traced in earlier chemists; but it does not appear to have acquired its peculiar chemical meaning till after Boerhaave's time. Boerhaave, however, is the writer in whom we first find a due apprehension of the peculiarity and importance of the Idea which it now expresses. When we make a chemical solution [N1], he says, not only are the particles of the dissolved body separated from each other, but they are closely united to the particles of the solvent. When aqua regia dissolves gold, do you not see, he says to his hearers, that there must be between each particle of the solvent and of the metal, a mutual virtue by which each loves, unites with, and holds the other (amat, unit, retinet)?
The opinion previously prevalent had been that the solvent merely separates the parts of the body dissolved: and most philosophers had conceived this separation as performed by mechanical operations of the particles, resembling, for instance, the operation of wedges breaking up a block of timber. But Boerhaave forcibly and earnestly points out the insufficiency of the conception. This, he says, does not account for what we see. We have not only a separation, but a new combination. There is a force by which the particles of the solvent associate to themselves the parts dissolved, not a force by which they repel and dissever them.
3. To Boerhaave is usually assigned also the credit of introducing the word “affinity” among chemists; but I do not find that the word is often used by him in this sense; perhaps not at all. [N2] But however this may be, the term is on many accounts well worthy to be preserved, as I shall endeavor to show. Other terms were used in the same sense during the early part of the eighteenth century. Thus when Geoffroy, in 1718, laid before the Academy of Paris his Tables of Affinities, which perhaps did more than any other event to fix the idea of affinity, he termed them “Tables of the Relations of Bodies” (“Talles des Rapports") speaking, however, also, of their “disposition to unite,” and using other phrases of the same import.
The term attraction, having been recommended by Newton as a fit word to designate the force which produces chemical combination, continued in great favor in England, where the Newtonian philosophy was looked upon as applicable to every branch of science. In France, on the contrary, where Descartes still reigned triumphant, “attraction,” the watch-word of the enemy, was a sound never uttered but with dislike and suspicion. In 1718 (in the notice of Geoffroy's Tables,) the Secretary of the Academy, after pointing out some of the peculiar circumstances of chemical combinations says, “Sympathies and attractions would suit well here, if there were such things.” (“Les sympathies, les attractions conviendroient bien ici, sielles etaient quelque chose.”) And at a later period, in 1731, having to write the éloge of Geoffroy after his death, he says, “He gave, in 1718, a singular system, and a Table of Affinities, or Relations of the different substances in chemistry. These affinities gave uneasiness to some persons, who feared that they were attractions in disguise, and all the more dangerous in consequence of the seductive forms which clever people have contrived to give them. It was found in the sequel that this scruple might be got over.”
This is the earliest published instance, so far as I am aware, in which the word “affinity” is distinctly used for the cause of chemical composition; and taking into account the circumstances, the word appears to have been adopted in France in order to avoid the word attraction, which had the taint of Newtonianism. Accordingly we find the word affinité employed in the works of French chemists from this time. Thus, in the Transactions of the French Academy for 1746, in a paper of Macquer's upon Arsenic, he says [N3], "One can easily account for these phenomena by means of affinity that different substances used in these combinations have with each other" [“On peut facilement rendre raison de ces phenomènes par le moyen des affinités que les differens substances qui entrent dans ces ombinaisons, ont les uns avec les autres”] and he proceeds to explain the facts by reference to Geoffroy's Table. And in Macquer's Elements of Chemistry, which appeared a few years later, the “affinity of composition” is treated of as a leading part of the subject, much in the same way as has been practiced in such books up to the present time. From this period the word appears to have become familiar to all European chemists in the sense of which we are now speaking. Thus, in the year 1758, the Academy of Sciences at Rouen offered a prize for the best dissertation on affinity. The prize was shared between Jean Philippe de Limbourg of Theux, near Liege, and Georges Le Sage of Geneva [N4]. About the same time other persons (Manherr [N5], Nicolai [N6], and others) wrote on the same subject, employing the same name.
|Libb Thims (right) explaining (Ѻ) the Bergman affinity table (1775), shown at top left (of sheet), along with the Bergman reaction diagrams (bottom left), Bergman symbols explained (top right), and Geoffroy affinity table (1718) (bottom right) at the econophysics and sociophysics conference at the 2013 University of Pitesti, Romania.|
Nevertheless, in 1775, the Swedish chemist Bergman, pursuing still further this subject of chemical affinities, and the expression of them by means of tables, returned again to the old Newtonian term; and designated the disposition of a body to combine with one rather than another of two others as elective attraction. And as his work on Elective Attractions had great circulation and great influence, this phrase has obtained a footing by the side of affinity, and both one and the other are now in common use among chemists.
4. I have said above that the term affinity is worthy of being retained as a technical term. If we use the word attraction in this case, we identify or compare chemical with mechanical attraction; from which identification and comparison, as I have already remarked no one has yet been able to extract the means of expressing any single scientific truth. If such an identification or comparison be not intended, the use of the same word in two different senses can only lead to confusion: and the proper course, recommended by all the best analogies of scientific history, is to adopt a peculiar term for that peculiar relation on which chemical composition depends. The word affinity, even if it were not rigorously proper according to its common meaning still, being simple, familiar, and well established in this very usage, is much to be preferred before any other.
But further, there are some analogies drawn from the common meaning of this word, which appear to recommend it as suitable for the office which it has to discharge. For common mechanical attractions and repulsions, the forces by which one body considered as a whole acts upon another external to it, are, as we have said, to be distinguished from those more intimate ties by which the parts of each body are held together. Now this difference is implied, if we compare the former relations, the attractions and repulsions, to alliances and wars between states, and the latter, the internal union of particles, to those bonds of affinity which connect the citizens of the same state with one another, and especially to the ties of family. We have seen that Boerhaave compares the union of two elements of a compound to their marriage; “we must allow,” says an eminent chemist [Pierre Duhem] of our own time [N7], “that there is some truth in this poetical comparison.” It contains this truth, that the two become one to most intents and purposes, and that the unit thus formed (the family) is not a mere juxtaposition of the component parts. And thus the idea of affinity as the peculiar principle of chemical composition, is established among chemists, and designated by a familiar and appropriate name.
5. Analysis is possible.—We must, however, endeavor to obtain a further insight into this idea, thus fixed and named. We must endeavour to extricate, if not from the idea itself, from the processes by which it has obtained acceptation and currency among chemists, some principles which may define its application, some additional specialties in the relations which it implies. This we shall proceed to do.
The idea of affinity, as already explained, implies a disposition to combine. But this combination is to be understood as admitting also of a possibility of separation. Synthesis implies analysis as conceivable: or to recur to the image which we have already used, divorce is possible when the marriage has taken place.
That there is this possibility, is a conviction implied in all the researches of chemists, ever since the true notion of composition began to predominate in their investigations. One of the first persons who clearly expressed this conviction was Mayow, an English physician, who published his Medico-Physical Tracts in 1674. The first of them, De Sale-Nitro et Spiritu Nitro-Aerio, contains a clear enunciation of this principle. After showing how, in the combinations of opposite elements, as acid and alkali, their properties entirely disappear, and a new substance is formed not at all resembling either of the ingredients, he adds [N8], “Although these salts thus mixed appear to be destroyed, it is still possible for them to be separated from each other, with their powers still entire." He proceeds to exemplify this, and illustrates it by the same image which I have already alluded to: “Salia acida a salibus volatilibus discedunt, ut *** sale fixo tartari, tanquam sponso magis idoneo, conjugium strictius ineunt." This idea of a synthesis which left a complete analysis still possible, was opposed to a notion previously current, that when two heterogeneous bodies united together and formed a third body, the two constituents were entirely destroyed, and the result formed out of their ruins f. And this conception of synthesis and analysis, as processes which are possible successively and alternately, and each of which supposes the possibility of the other, has been the fundamental and regulative principle of the operations and speculations of analytical chemistry from the time of Mayow to the present day.
6. Affinity is elective.—When the idea of chemical affinity, or disposition to unite, was brought into view by the experiments and reasonings of chemists, they found it necessary to consider this disposition as elective: each element chose one rather than another of the elements which were presented to it, and quitted its union with one to unite with another which it preferred. This has already appeared in the passage just quoted from Mayow. He adds in the same strain, “I have no doubt that fixed salts choose one acid rather than another, in order that they may coalesce with it in a more intimate union.”—“Nullus dubito salia fixa acidum unum pre aliis eligere, ut *** eodem arctiore unione coalescant." The same thought is expressed and exemplified by other chemists: they notice innumerable cases in which, when an ingredient is combined with a liquid, if a new substance be immersed which has a greater affinity for the liquid, the liquid combines with the new substance by election, and the former ingredient is precipitated. Thus Stahl says [N9], “In spirit of nitre dissolve silver; put in copper and the silver is thrown down; put in iron and the copper goes down; put in zinc, the iron precipitates; put in volatile alkali, the zinc is separated; put in fixed alkali, the volatile quits its hold.”—As may be seen in this example, we have in such cases, not only a preference, but a long gradation of preferences. The spirit of nitre will combine with silver, but it prefers copper; prefers iron more; zinc still more; volatile alkali yet more; fixed alkali the most. The same thing was proved to obtain with regard to each element; and when this was ascertained, it became the object of chemists to express these degrees of preference, by lists in which substances were arranged according to their disposition to unite with another substance. In this manner was formed Geoffroy's Table of Affinities (1718), which we have already mentioned. This Table was further improved by other writers, as Gellert (1751) and Limbourg (1761). Finally Bergman improved these Tables still further, taking into account not only the order of affinities of each element for others, but the sum of the tendencies to unite of each two elements, which sum, he held, determined the resulting combination when several elements were in contact with each other.
7. As we have stated in the history [N10], when the doctrine of elective affinities had assumed this very definite and systematic form, it was assailed by Berthollet, who maintained, in his Essai de Statique Chimique, (1803,) that chemical affinities are not elective:—that, when various elements are brought together, their combinations do not depend upon the kind of elements alone, but upon the quantity of each which is present, that which is most abundant always entering most largely into the resulting compounds. It may seem strange that it should be possible, at so late a period of the science, to throw doubt upon a doctrine which had presided over and directed its progress so long. Proust answered Berthollet, and again maintained that chemical affinity is elective. I have, in the History, given the judgment of Berzelius upon this controversy. "Berthollet," he says, "defended himself with an acuteness which makes the reader hesitate in his judgment; but the great mass of facts finally decided the point in favour of Proust." I may here add the opinion pronounced upon this subject by Dr. Turnerf. "Bergman erred in supposing the result of the chemical action to be in every case owing to elective affinity [for this power is modified in its effects by various circumstances]: but Berthollet ran into the opposite extreme in declaring that the effects formerly ascribed to that power are never produced by it. That chemical attraction is exerted between different bodies with different degrees of energy, is, I apprehend, indisputable." And he then proceeds to give many instances of differences in affinity which cannot be accounted for by the operation of any modifying causes. Still more recently, M. Dumas has taken a review of this controversy; and, speaking with enthusiasm of the work of Berthollet, as one which had been of inestimable service to himself in his early study of chemistry, he appears at first disposed to award to him the victory in this dispute. But his final verdict leaves undamaged the general principle now under our consideration, that chemical affinity is elective. "For my own part," he says*, "I willingly admit the notions of Berthollet when we have to do with acids or with bases, of which the energy is nearly equal: but when bodies endued with very energetic affinities are in presence of other bodies of which the affinities are very feeble, I propose to adopt the following rule: In a solution, everything remaining dissolved, the strong affinities satisfy themselves, leaving the weak affinities to arrange matters with one another. The strong acids take the strong bases, and the weak acids can only unite with the weak bases. The known facts are perfectly in accordance with this practical rule." It is obvious that this recognition of a distinction between strong and weak affinities, which operates to such an extent as to determine entirely the result, is a complete acknowledgement of the elective nature of affinity, as far as any person acquainted with chemical operations could contend for it. For it must be allowed by all, that solubility, and other collateral circumstances, influence the course of chemical combinations, since they determine whether or not there shall take place that contact of elements without which affinity cannot possibly operate.
8. Affinity is Definite as to Quantity—In proportion as chemists obtained a clearer view of the products of the laboratory as results of the composition of elements, they saw more and more clearly that these results were definite; that one element not only preferred to combine with another of a certain kind, but also would combine with it to a certain extent and no further, thus giving to the result not an accidental and variable, but a fixed and constant character. Thus salts being considered as the result of the combination of two opposite principles, acid and alkali, and being termed neutral when these principles exactly balanced each other, Rouelle (who was Royal Professor at Paris in 1742) admits of neutral salts with excess of acid, neutral salts with excess of base, and perfect neutral salts. Beaume maintained" against him that there were no salts except those perfectly neutral, the other classes being the results of mixture and imperfect combination. But this question was not adequately treated till chemists made every experiment with the balance in their hands. When this was done, they soon discovered that, in each neutral salt, the proportional weights of the ingredients which composed it were always the same. This was ascertained by Wenzel, whose Doctrine of the Affinities of Bodies appeared in 1777. He not only ascertained that the proportions of elements in neutral chemical compounds are definite, but also that they are reciprocal; that is, that if A, a certain weight of a certain acid, neutralize m, a certain weight of a certain base, and B, a certain weight of a certain other acid, neutralize n, a certain weight of a certain other base; the compound of A and n will also be neutral; as also that of B and m. The same views were again presented by Richter in 1792, in his Principles of the Measure of Cle. mical Elements. And along with these facts, that of the combination of elements in multiple proportions being also taken into account, the foundations of the Atomic Theory were laid; and that theory was propounded in 1803 by Mr. Dalton. That theory, however, rests upon the idea of substance, as well as upon that idea of chemical affinity which we are here considering; and the discussion of its evidence and truth must be for the present deferred.
9. The two principles just explained, that affinity is definite as to the kind, and as to the quantity of the elements which it unites, have here been stated as results of experimental investigation. That they could never have been clearly understood, and therefore never firmly established, without laborious and exact experiments, is certain; but yet we may venture to say that being once known, they possess an evidence beyond that of mere experiment. For how, in fact, can we conceive combinations, otherwise than as definite in kind and quantity? If we were to suppose each element ready to combine with any other indifferently, and indifferently in any quantity, we should have a world in which all would be confusion and indefiniteness. There would be no fixed kinds of bodies; salts, and stones, and ores, would approach to and graduate into each other by insensible degrees. Instead of this, we know that the world consists of bodies distinguishable from each other by definite differences, capable of being classified and named, and of having general propositions asserted concerning them. And as we cannot conceive a world in which this should not be the case, it would appear that we cannot conceive a state of things in which the laws of the combination of elements should not be of that definite and measured kind which we have above asserted. This will, perhaps, appear more clearly by stating our fundamental convictions respecting chemical composition in another form, which I shall, therefore, proceed to do.
10. Chemical Composition determines Physical Properties.—However obscure and incomplete may be our conception of the internal powers by which the ultimate particles of bodies are held together, it involves, at least, this conviction:—that these powers are what determine bodies to be bodies, and therefore contain the reason of all the properties which, as bodies, they possess. The forces by which the particles of a body are held together, also cause it to be hard or soft, heavy or light, opake or transparent, black or red; for if these forces are not the cause of these peculiarities, what can be the cause? By the very supposition which we make respecting these forces, they include all the relations by which the parts are combined into a whole, and therefore they, and they only, must determine all the attributes of the whole. The foundation of all our speculations respecting the intimate constitution of bodies must be this, that their composition determines their properties.
Accordingly we find our chemists reasoning from this principle with great confidence, even in doubtful cases. Thus Davy, in his researches concerning the diamond, says: “That some chemical difference must exist between the hardest and most beautiful of the gems and charcoal, between a non-conductor and a conductor of electricity, it is scarcely possible to doubt: and it seems reasonable to expect that a very refined or perfect chemistry will confirm the analogies of nature; and show that bodies cannot be the same in their composition or chemical nature, and yet totally different in their chemical properties.” It is obvious that the principle here assumed is so far from being a mere result of experience, that it is here appealed to prove that all previous results of experience on this subject must be incomplete and inaccurate; and that there must be some chemical difference between charcoal and diamond, though none had hitherto been detected.
11. In what manner, according to what rule, the chemical composition shall determine the kind of the substance, we cannot reasonably expect to determine by mere conjecture or assumption, without a studious examination of natural bodies and artificial compounds. Yet even in the most recent times, and among men of science, we find that an assumption of the most arbitrary character has in one case been mixed up with this indisputable principle, that the elementary composition determines the kind of the substance. In the classification of minerals, one school of mineralogists have rightly taken it as their fundamental principle that the chemical composition shall decide the position of the mineral in the system. But they have appended to this principle, arbitrarily and unjustifiably, the maxim that the element which is largest in quantity shall fix the class of the substance. To make such an assumption is to renounce, at once, all hope of framing a system which shall be governed by the resemblances of the things classified; for how can we possibly know beforehand that fifty-five per cent of iron shall give a substance its predominant properties, and that forty-five per cent. shall not? Accordingly, the systems of mineralogical arrangement which have been attempted in this way, (those of Haüy, Phillips, and others) have been found inconsistent with themselves, ambiguous, and incapable of leading to any general truths.
12. Thus the physical properties of bodies depend upon their chemical composition, but in a manner which a general examination of bodies with reference to their properties and their composition can alone determine. We may, however, venture to assert further, that the more definite the properties are, the more distinct may we expect to find this dependence. Now the most definite of the properties of bodies are those constant properties which involve relations of space: that is, their figure. We speak not, however, of that external figure, derived from external circumstances, which, so far from being constant and definite, is altogether casual and arbitrary; but of that figure which arises from their internal texture, and which shows itself not only in the regular forms which they spontaneously assume, but in the disposition of the parts to separate in definite directions and no others. In short, the most definite of the properties of perfect chemical compounds is their crystalline structure; and therefore it is evident that the crystalline structure of each body, and the forms which it affects, must be in a most intimate dependence upon its chemical composition.
Here again we are led to the brink of another theory; —that of crystalline structure, which has excited great interest among philosophers ever since the time of Haüy. But this theory involves, besides that idea of chemical composition with which we are here concerned, other conceptions which enter into the relations of figure. These conceptions, governed principally by the idea of Symmetry, must be unfolded and examined before we can venture to discuss any theory of crystallization: and we shall proceed to do this as soon as we have first duly considered the Idea of Substance and its consequences.
Note: in later parts of the book, Whewell touches on what he terms “natural affinity”. (Ѻ)
Note: on his [N2], in point #3 (above), Whewell credits Herman Boerhaave (c.1720s) as having introduced to chemists the word "affinity"; James Partington (1937), however, credits Albertus Magnus (c.1250) with the introduction of the term.
N1. Elementa Chemia. Lugd. Bat. 1732, p. 677.
N2. See DUMAs, Leçons de Philos. Chim, p. 364. REEs' Cyclopædia, Art. Chemistry. In the passage of Boerhaave to which I refer above, affinitas is rather opposed to, than identified with, chemical combination. When, he says, the parts of the body to be dissolved are dissevered by the solvent, why do they remain united to the particles of the solvent, and why do not rather both the particles of the solvent and of the dissolved body collect into homogeneous bodies by their affinity? denuo se affinitate suae naturae colligant in corpora homogenea? And the answer is, because they possess another force which counteracts this affinity of homogeneous particles, and makes compounds of different elements. Affinity, in chemistry, now means the tendency of different kinds of matter to unite: but it appears, as I have said, to have acquired this sense since Boerhaave's time.
N3. A. P. 1746, p.201.
N4. Thomson’s Chemistry, iii. 10. Limbourg's Dissertation was published at Liege, in 1761; and Le Sage's at Geneva.
N5. Dissertatio de Affinitate Corporum. Vindob. 1762.
N6. Progr. I. II. de Affinitate Corporum Chimica. Jen. 1775, 1776.
N7. DUMAs, Leçons de Phil. Chim, p. 363.
N8. Cap. Xiv., pg. 233
N9. Zymotechnica, 1697, pg. 117.
N10. Hist. Ind. Sci., iii. 115.
1. Whewell, William. (1840). The Philosophy of the Inductive Sciences: Founded Upon Their History, Volume One (Book 4, §2: The Establishment and Development of the Idea of Chemical Affinity, pgs. 373-87; "si amor dicendus copulae cupido", pg. 375; Dumas, pg. 379). John W. Parker.