Popular Science Monthly/Volume 34/April 1889/The Chemical Elements
HISTORY OF THE CONCEPTION WHICH THIS TERM INVOLVES.
By JOSIAH PARSONS COOKE, LL. D.,
ERVING PROFESSOR OF CHEMISTRY AND MINERALOGY IN HARVARD UNIVERSITY.
THE intellectual force of Aristotle ruled in chemistry even longer than in other departments of physical science. In mechanics and astronomy the dogmas of Aristotle were effectually laid by Galileo early in the seventeenth century; but his doctrine of the four elements—in one form or other—was accepted in chemistry to the close of the eighteenth century. The wonderful history of the philosophy of the great Stagirite—a philosophy which ruled the intellectual world in physics as well as in metaphysics for more than twenty centuries—is constantly referred to as an illustration of the vices of speculative thought when not based on experimental evidences; and, undoubtedly, the aberrations of many of his later disciples justify this opinion. Among these Kepler is especially conspicuous, for he, by applying the doctrines in a most grotesque and absurd manner, did not a little to bring Aristotle's philosophy of mechanics into contempt. Nevertheless, Aristotle himself was for his time an acute observer, as his writings abundantly indicate; and his philosophical views are brought forward rather to justify his conclusions than as the basis of his inferences. So it is with the doctrine of the four elements. Earth, water, air, and fire were obviously to him the essences, or, to use a later and more descriptive word, the "substantia" of four conditions of matter. Three of these we recognize as clearly as he did; and the fourth, fire, which he regarded as a more sublimated condition than air, and thought he actually saw in the upward motion of flames, has its modern representative in Mr. Crookes's fourth condition of matter.
Since Aristotle regarded motion as an attribute of inanimate as well as of living bodies—a stone falling for the same reason that a fish swims—and as he noticed that while water and stones tend to fall, flame and air tend to rise, he regarded the last as having a natural motion upward, and the first as having a natural motion downward; and thus to him specific levity seemed as much a direct inference of observation as specific gravity. By this inherent motion the four elements appeared to strive to separate, and each to tend to its own place—fire taking the highest place, air the next, water the next, and earth the lowest. The reasons urged in support of these conclusions appear to us absurd enough.
By Aristotle, as by other Greek philosophers, the contrasts emphasized by language were regarded as fundamental distinctions in nature, or first principles, which they made the basis of discussion, and from which they sought to deduce general truths. Aristotle enumerates ten such principles as enunciated by the Pythagoreans—limited and unlimited, odd and even, one and many, right and left, male and female, rest and motion, straight and curved, light and darkness, good and evil, square and oblong—and from oppositions of this kind he deduced his doctrine of the four elements.
"We seek," Aristotle writes, "the principles of sensible things, that is, of tangible bodies. We must take, therefore, not all the contrarieties of quality, but those only which have reference to the touch. Thus, black and white, sweet and bitter, do not differ as tangible qualities, and must therefore be rejected from our consideration. Now, the contrarieties of quality which refer to the touch are these: hot, cold; dry, wet; heavy, light; hard, soft; unctuous, meager; rough, smooth; dense, rare." Then, after rejecting all but the first four of these, either because they are not active and passive qualities, or because they are combinations of the first four, and concluding for these reasons that the four retained must be elements, he proceeds: "Now, in four things there are six combinations of two; but the combinations of two opposites, as hot and cold, must be rejected. We have, therefore, four elementary combinations which agree with the four apparently elementary bodies: fire is hot and dry; air is hot and wet (for steam is air); water is cold and wet; earth is cold and dry."
In a similar way, by considering light as opposite to heavy, Aristotle justifies his conclusion that levity is a quality of a body, and that bodies are absolutely heavy or absolutely light. "Former writers," he says, "have considered heavy and light relatively only—taking cases where both things have weight, but one is lighter than the other, and they imagined that in this way they defined what was absolutely heavy and light." Fire and air, according to Aristotle, were absolutely light, with fire the lighter of the two; while water and earth were absolutely heavy, with earth the heavier of the two. In another place he writes, "Heavy and light are, as it were, the embers or sparks of motion"; and hence he concluded that the tendency of light bodies to rise, like the tendency of heavy bodies to fall, was an inherent quality.
Subsequently Aristotle recognized a fifth element in nature. In his book "On the Heavens" he wrote: "The simple elements must have simple motions; and thus fire and air have their natural motions upward, and water and earth have their natural motions downward. But, besides these motions, there is motion in a circle, which is unnatural to these elements, but which is a more perfect motion than the other, because a circle is a perfect line, and a straight line is not; and there must be something to which this motion is natural. From this it is evident that there is some essence of body different from those of the four elements, more divine than those and superior to them. If things which move in a circle move contrary to nature, it is marvelous or rather absurd that this, the unnatural motion, should alone be continuous and eternal; for unnatural motions decay speedily. And so from all this we must collect that, besides the four elements which we have here and about us, there is another removed far off, and the more excellent in proportion as it is more distant from us." This element was called the quinta essentia by Latin writers, and the word quintessence in our own language frequently brings to mind this singular conception, which, although so absurd to us, held for ages a wonderful control over the human mind.
It is not, however, our purpose to trace the influence of the dynamical conceptions of Aristotle on the development of physical science, interesting and instructive as such a study would be. We are here dealing only with the conception of an element or principle of material bodies, also involved in this reasoning; and it is obvious that this early conception of an element was not that of a definite substance—as we now understand the word substance—that is, something subsistens per se—but rather that of the essentia or substantia which were supposed to underlie the external attributes of bodies, and of which these last were merely accidents. Earth was the underlying principle of all solid bodies, whose multifarious forms were as familiar to Aristotle as to us. So all liquid bodies were forms of water, and all aëriform bodies manifestations of the all-diffusive air; and the ancients, at times even more acute than ourselves, made distinctions between conditions, both of water and air, which we know are not essential.
We know that flame is simply intensely heated gas rising in a denser atmosphere; but it was perfectly natural that the ancients should regard such a startling effect as a manifestation of a fourth condition of matter still lighter and more subtile than air, and the conception of fire as a fundamental principle of nature once formed, the phenomena of combustion appeared to them as direct evidences of the escape of this principle of fire from the burning bodies.
The famous theory of phlogiston, advanced by Becher and Stahl during the seventeenth century, was simply a development of these views without any essential change. Phlogiston was merely a new name for the fourth element of Aristotle. As by Aristotle all combustible bodies were assumed to hold the principle of fire. so, on the new theory, they were regarded as compounds of phlogiston, and, in burning, the phlogiston was supposed to escape into the atmosphere. The ease with which such metals as zinc, iron, lead, and tin burn under certain conditions was well known to the chemists of that period, and hence all metals were regarded as largely composed of phlogiston; and when it was shown that the oxides, then called calces, resulting from the burning, weighed more than the metal burned, the facts were cited to prove that phlogiston was specifically light, and therefore, when removed from a body, added to its weight.
It has been said that the increase of weight resulting from burning and other forms of oxidation was not recognized until Lavoisier introduced the balance into chemical investigations at the close of the last century; but, although such phenomena could not be formulated under a general principle until after the discovery of oxygen in 1774 (nearly simultaneously both by Priestley and by Scheele), the fact that the so-called calces resulting from the burning of the metals weigh more than the metals was well known to metallurgists from a much earlier period. Thus Lémery, who died in 1715, in his well-known treatise on chemistry, describes the increase of weight attending the calcination both of tin and lead; and Boerhaave, a famous Dutch physician and chemist of the same period, thus describes the calcination of lead: "And if, while the lead is in fusion, it be kept continually stirring with a spatula, it turns into a red powder called minium, or red lead, in which operation this is further observable that the lead augments in weight."
During the eighteenth century the theory of phlogiston became modified by the increasing knowledge of the definiteness of chemical combination. Like the other constituents of a body, it was held that the phlogiston in combustibles must be united in definite proportions. So, moreover, when, leaving the fuel in the process of combustion, phlogiston entered into union with the air, it could only be absorbed by the atmosphere up to a certain limit. Hence, a candle soon goes out if burned in a confined vessel; because, after the air is saturated with phlogiston, no more can escape from the combustible. Priestley called oxygen gas, when first discovered, dephlogisticated air, because he ascribed its wonderful power of sustaining combustion to the absence of phlogiston, which oxygen gas could therefore absorb to a proportionally great extent. On the other hand, hydrogen was called phlogisticated air; and Cavendish, when he first isolated this exceedingly light and combustible gas, thought he had discovered phlogiston itself.
As has been already intimated, Aristotle's doctrine of the chemical elements was, in some form or other, received by students of chemistry down to the time of Lavoisier, and the aims and practice of alchemy, which for many centuries was the only phase of chemistry studied, were wholly in harmony with this conception. If the metals were all manifestations of the same underlying essence, and differed only in the accidents of external qualities, it was reasonable to suppose that these accidents might be changed. The alchemists were often intelligent men, and knew as well as ourselves that "all is not gold that glitters"; but the resemblances to the precious metals which they sometimes obtained by their empirical methods were sufficient to stimulate effort. They also clearly saw that the value of the prize they sought would vanish in their keeping the moment the secret became known; but this only led them, as it does so many manufacturers of the present day, to invest their processes with all possible mystery, to conceal known facts beneath non-essentials, and to adopt a conventional and highly figurative language for communicating with each other, so that, even with our knowledge of chemistry, the writings of the alchemists are for the most part an unintelligible jargon. Still, their hopes were based on what they regarded as sound philosophy; and, although their efforts were frequently exposed to ridicule on the ground of ill success, no convincing objections were ever raised to the philosophy by which they were guided. That the aims of the alchemists must have appeared reasonable to thinking men is shown by the fact that, even at a late period in the history of this apparent delusion. Sir Isaac Newton, whose scientific sobriety can not be questioned, devoted a great deal of time to experiments on the transmutation of the metals.
During the two thousand years through which the doctrine of a few elementary principles of nature prevailed, the precise form which the elements assumed naturally varied with the general point of view of the students at the time, although for the most part philosophical writers adhered to the statement of Aristotle. By many of the alchemists mercury, sulphur, and salt were regarded as fundamental principles, because the crude materials under these names played such an important part in the hermetic art. Here, however, it was not these crude materials which were regarded as the elements of matter, but sublimated forms of these substances, known as the mercury and sulphur of the philosophers; and for a long time the conceit was cherished that, if once the elemental mercury and sulphur could be isolated, all metals, and, of course, gold and silver among the number, could be manufactured by mixing these elements in the right proportions. Later, when chemistry assumed a pharmaceutical character, the elements were often said to be water, spirit, oil, salt, and earth, of which the first three were regarded as active and the last two as passive principles. These elements, again, were not definite substances, but merely classes of products obtained by distillation, the active principles being those that passed over and the passive principles those that remained behind in this process—a process which at the time had become the typical process of chemistry, and the chemists of this period are always represented in paintings with a retort or alembic, as were the alchemists of an earlier period with a furnace and crucible. This last enumeration of elements is not so different from that of the alchemists as would at first sight seem, for mercury was regarded as the most active of the spirits, and sulphur as one of the oils. Moreover, the distinction between fixed and volatile oils, which dates from this period, shows the generic character of the elements then accepted.
In his "Œdipus Cymicus," first published about the middle of the seventeenth century, Becher, the author of the theory of phlogiston, comes back to the elements of Aristotle, and in this he is followed by Stahl, who elaborated the same theory a generation later. To give an idea of the confusion of thought on this subject, even at a comparatively late period, I will quote from the "Cours de Chimie, par M. Lémery, nouvelle édition, Paris, 1756," a work which remained one of the chief authorities on chemistry down to the time of Lavoisier. I translate freely from the French:
"The first element of compound bodies which we must accept is a universal spirit, which, being universally diffused, produces different results according as it is held in different matrices or pores of the earth; but as this principle is somewhat metaphysical and can not be perceived by the senses, we must distinguish in addition certain elements which are perceptible. I shall name those commonly accepted.
"As chemists in analyzing different compounds have found five kinds of substances, they have concluded that there are five principles of material things—water, spirit, oil, salt, and earth. Of these five there are three which are active principles—spirit, oil, and salt; and two passive—water and earth. The first are called active, because, being endowed with rapid motion, they determine the active qualities of the products into which they enter; and the second are called passive, because, being at rest, they only serve to diminish the vivacity of the active principles."
Then follows a more precise definition of the several principles enumerated, to which in part I have already referred. After this, Lémery remarks:
"The term principle of chemistry must not be taken in an exact sense, for the substances to which we have given this name are principles only relating to our knowledge, and so far as we have been unable to go further in the division of bodies. But we can well understand that these principles may be further divisible into an infinity of parts, which should more properly be called principles. We understand, then, by principles simply such substances as have been separated and divided so far as our feeble efforts are capable of doing." Here is a glimmering of scientific principles. And so again in this sentence:
"Some modern philosophers would persuade us that it is uncertain whether the products we draw from compounds, and which we call principles of chemistry, really exist as such in the compounds. They say that fire, rarefying matter in the process of distillation, is capable of giving an entirely different arrangement to the parts from that which existed before, and may thus form the salt, oil, and other products which are the results of the process."
Lémery himself died in 1715, so that the edition of his work from which we quote was published over forty years after his death, showing that in the slow progress of knowledge at that time the life of a scientific treatise was far longer than it is now. The editor of the new edition adds copious notes, in which he comments on some of the absurdities of his author, plainly indicating that progress toward clearer views was constantly being made; but, at the same time, his own remarks are equally amusing, and give abundant evidence of the utter confusion of thought which still prevailed. To appreciate how great a work Lavoisier accomplished, it is only necessary to read a few pages (more would be intolerable), both of this treatise of Lémery and also of the "New Method of Chemistry" of Boerhaave, the two great standard works on the science of the eighteenth century, both in large quarto volumes. These are far less repulsive than the chemical writings of the previous century, which often dwelt at great length on illustrations of chemical processes from the relations of the sexes. They are less mystical, and frequently describe acute observations of phenomena; but they are equally deficient in scientific spirit, full of crudities and empiricisms of the most trivial kind, and this at a period when the mathematical sciences had attained much of the elegance of form of our own day. Lavoisier is known to us chiefly as the discoverer of the true theory of combustion, but he was truly the father of modern chemistry, and his claim to our regard rests more than anything else on the fact that he gave to the subject for the first time a definite and rigid scientific form. It will help you to appreciate the entire change of conception introduced by Lavoisier if I quote from Fourcroy's "Chemical Philosophy," third edition, 1806, the following significant passage. Fourcroy was a contemporary of Lavoisier, although twelve years younger. Lavoisier, as is well known, fell a victim of the Reign of Terror during the French Revolution in 1794. Fourcroy, more fortunate than his greater colleague, passed through this fearful period unharmed, although he was a member of the Constituent Assembly, and after the fall of Robespierre acted as Secretary of Public Instruction. He was made senator by Napoleon, and died full of honors in 1809, living until the decomposition of the alkalies and alkaline earths had become accomplished facts. As before, I translate from the French very freely:
"Since the revolution effected in chemistry between 1774 and 1784" (the period of Lavoisier's active scientific work) "by the new discoveries which have entirely changed the face of the science, many of the former erroneous and arbitrary distinctions have been given up. The term principle is no longer used except in a very general sense, and with the understanding that it applies to different sorts of bodies, some of them simple and some of them compound, depending on the nature of the materials from which they come and on the method of analysis used. All chemists agree to-day that if by principles or elements we understand the original and simple bodies which constitute the primitive molecules of substances, such bodies are wholly unknown, either as regards their number or their properties, and that in discussing them we are yielding to theories as useless as those of monads or atoms. They further agree that if we confine the word elements to the last products of analysis which can not be subdivided by analytical means, we must exclude from this class of bodies both the so-called principles of the elder chemists and the four elements of Aristotle, as many of these are compound substances, and we must accept a very much larger number of elements than formerly, for we are acquainted with more than thirty substances which can not be decomposed.
"From the results of numerous and exact analyses chemists know, first, that all natural substances may be divided into simple and compound substances; secondly, that the true distinction of primary or simple substances is ability to resist decomposition, so that the word simple is synonymous with the word undecomposable; thirdly, that by compounds we signify substances which are susceptible of analysis, or from which we can extract materials more simple, or of which the complexity of composition diminishes in degree as the analysis is extended; fourthly, that although compounds of the same class may differ greatly among themselves, it is sufficient for comparison and gives us an exact distinction if we divide them into binaries or compounds formed of two elements, ternaries or compounds formed of three elements, quaternaries or compounds formed of four elements, quinaries, sextaries, etc., according as the number of the constituent elements increases; fifthly, that the number of the constituent principles or components is not the only cause of the differences which distinguish compounds, but that the proportions in which these elements are united, and perhaps also the mode of their union, are other causes of these differences.
"Thus the whole doctrine of the pretended elements, or of the principles of things, or of their components, or of the compositions of different orders of compounds, is now reduced to conceptions as simple as they are precise. There are no hypotheses or useless distinctions or erroneous abstractions in the present ideas of chemists, and the obscurity which formerly reigned in this part of the science has wholly disappeared, and at the same time we have got rid of a source of vague and endless discussions. We have no longer to dwell in the schools on useless questions about a primitive matter and its relations; on whether there are four, three, two, or only a single element; on the pretended relations of the elements among themselves; on their transformation, or on the change of one into another. All these dreams of a sham speculative philosophy have vanished before the facts discovered by the experimental method; and the five propositions enunciated above, as simple as they are true, are data on which we can now securely build."Turning, now, to Lavoisier's own "Traité élémentaire de Chimie," which must be regarded as the "Principia" of chemical science, we find, for the first time in the history of the subject, a list of twenty-five definite substances distinguished as elementary on the sole basis that they had as yet never been analyzed. This list is given in the first column of the table which we reproduce in translation on the following page, on account of its very great historical interest. Still, there is even here an obvious survival of Aristotle and the phlogiston theory, both in what the list includes and in what it omits. The first name on the list is caloric, and three of the other elements are the muriatic, fluoric, and boracic radicals, which, though not yet isolated, appear to Lavoisier so distinctly typified and foreshadowed that he does not hesitate to name them in this list. These radicals, it must be noticed, were radicals which, united to oxygen, would form respectively hydrochloric, hydrofluoric, and boracic acid, so that in the last case only were Lavoisier's expectations realized in the form which he expected. Indeed, the radical of muriatic acid, chlorine, was then a well-known substance, having been discovered by Scheele in 1774, but so little did it answer to the expected radical that it was regarded by Lavoisier as an oxide, and named by him "acide muriatique oxygéné," and under this name appears in this very table (translated oxidized muriatic acid). What we know as chlorine gas was classed by Lavoisier as the fourth degree of oxidation of his assumed muriatic radical, while muriatic acid itself was the
TABLE OF THE BINARY COMBINATIONS OF OXYGEN WITH OXIDABLE AND ACIDIFIABLE METALLIC AND NON-METALLIC SUBSTANCES.
third degree of oxidation of the same radical. It was left for Davy to demonstrate the elementary nature of chlorine and to discover the true relations of the hydrogen acids. Lavoisier regarded oxygen as the universal acidifying principle, and the facts known in his day admitted of this interpretation; and it is interesting to see how they were worked up in the table; but when a class of acids containing no oxygen came to be clearly recognized, they proved a serious embarrassment to the Lavoisierian system as it was developed by Berzelius and his associates.
With the exception of caloric and two of the radicals above referred to, Lavoisier's list of elements includes no substance not regarded as elementary at the present day; but the list is as remarkable for what it omits as for what it includes. There were then known, and had been known for a long time, two very well marked classes of bodies called alkalies and earths which readily combined with acids to form salts. In this respect these bodies closely resembled the known metallic oxides, as they did also in most cases in their general appearance, and they were classed by Lavoisier with the oxides under the general term of "bases salifiables." Still, they had never been decomposed, and, according to the spirit of Lavoisier's philosophy, ought to have been classed among elementary substances; but Lavoisier's classificatory instinct was altogether too acute to permit him to fall into any such error. He enumerates these bodies, and, although he speaks doubtfully in regard to them, he never for a moment questions their compound nature. In regard to the earths he says their composition is wholly unknown, implying, of course, that they were compounds, and under the head of "Des Substances métalliques" is this significant paragraph:
"It is probable that we only know a part of the metallic substances which exist in nature. All those, for example, which have more affinity for oxygen than for carbon can not be reduced or brought to a metallic state, and must appear to us as oxides which we mistake for earths. It is very probable that baryta, which we have classed as an earth, is a case in point. When experimented upon, it exhibits characters which closely approach those of metallic substances. It may be, indeed, that all the substances to which we give the name of earths are only metallic oxides that can not be reduced by the means which we use."
It will be noticed that the alkalies are not included under this remark, for their active qualities are very different from those of an insipid, earthy-looking, metallic oxide; and their resemblance to ammonia, the volatile alkali, a known compound of nitrogen, was constantly a confusing circumstance. Lavoisier discusses the question whether potash and soda pre-exist as such in the plants from whose ashes they are procured, and makes the suggestion that they may result from the combined action of the oxygen and nitrogen of the atmosphere on the organic materials in the process of burning. Fourcroy goes still further. In his work on "Chemical Philosophy," to which I have referred, he writes (translation):
"We do not understand the composition of potash. It has been suspected that it might result from a union of lime with nitrogen, because it is often found in vegetables mixed with this earth; but this theory, which I brought forward some fifteen years ago, has not been proved by any positive fact." It is interesting to go back and watch this groping in the dark for what is now positive knowledge, but the experience may teach us many a valuable lesson, and will at least help us to realize the intense enthusiasm with which, on October 6, 1807, Davy saw metallic globules running from a lump of caustic potash under the influence of the current of his new voltaic battery.
With this great achievement of Davy the formative period of the Lavoisierian system of chemistry may be said to have closed; but in this connection it is amusing to notice that in a chemical text-book studied in Harvard College by the class of 1815, and given me by the late Hon. John G. Palfrey, of that class, the alkalies and earths are included in the list of chemical elements, and Davy's discovery is only briefly referred to in a note.
Immediately after Davy's short but brilliant career, the science of chemistry took the form which it retained for nearly fifty years—a form in which it was first studied by all the older men of the present generation. The form was essentially that given by Lavoisier, and its chief merit was the simplicity of the classification, and the admirable nomenclature in which this classification was expressed. This nomenclature, which is to a great extent still retained, although the terms have lost most of their original significance, was devised by Lavoisier, with the co-operation of several of his associates, and adopted with the sanction of the French Academy of Sciences. It was a masterly production, and very greatly strengthened the hold which the system acquired at all the great centers of learning. The general features of the Lavoisierian system can be stated in few words.
Oxygen, which constitutes at least one half of the earth's crust, is the common cement by which all the elementary parts are held together. It is the universal acidifying principle, and the salifiable bases owe their peculiar relations to the same element as well. The elements may be divided into metals and non-metallic substances. The direct compounds of the non-metals with oxygen in different proportions are acids, while the compounds of the metals with oxygen are salifiable bases, and the compounds of the acids and bases are salts; and simple salts may still further combine with each other to form double salts. Thus, beginning with the elements, combination proceeded, pair and pair, until all terrestrial products were educed. The members of each class of these products were designated by specific names, regularly formed and easily remembered.
Such a simple system could easily be comprehended and presented in such works as that of the late Dr. Turner, and, illustrated by the brilliant phenomena of combustion, had a great charm. I can remember most distinctly the impression it made on me as a boy, and I have heard many learned men, among others my late colleague, Dr. Asa Gray, speak in the most glowing terms of the impression it made on them.
Lavoisier himself regarded his system as perfectly true to nature, and often affirms that he accepts no conclusion not based on experimental evidence; but, with the progress of knowledge, the system soon became highly artificial. Indeed, it never would have been formulated had not its author's vision been restricted to the narrow field that had been cultivated in his time. As investigation extended, the class of hydrogen acids, and their products, which Lavoisier had hidden away under a mistaken interpretation of their constitution, assume an ever-increasing prominence; and the system was doomed when Berzelius felt obliged to withdraw this class of bodies from the general scheme, and place them by themselves in a special division, which he called the haloids. Then after a time it appeared that the simple oxides of the elements had neither acid nor basic properties in themselves, and only acquired active qualities of either kind when united with water; and that hydrogen and not oxygen was the acidifying principle. Moreover, multitudes of compounds were discovered in whose production oxygen took no part whatever, and, although attempts were made to classify these on the same general dualistic plan, assuming that sulphur, chlorine, or one of the allied elements might act in place of oxygen as a general binding agent in a chemical combination, yet the attempts were obvious failures.
Before I became a teacher of chemistry, in 1849, it had already become evident that Lavoisier's definition of a chemical element, as a substance that could not be decomposed, must be modified; or, at least, that even if our actual processes of analysis could not go beyond the substances regarded as elementary, the philosophy could not possibly be thus restricted. Many facts previously known but overlooked, and other facts then first discovered which exhibited the old facts in a stronger light, all combined to show clearly that the same chemical element might appear under the guise of different substances. By burning a gem in oxygen gas, Davy had proved that diamond was pure carbon; and when it was also shown that the iron in graphite was an accidental impurity, it appeared that carbon was known under three forms, diamond, graphite, and charcoal. Sulphur, in like manner, was found to crystallize in two wholly incompatible forms, and under these different phases showing such marked differences of qualities that they must be regarded as distinct substances. In 1845 Schrötter proved that what had before been known as red phosphorus, and thought to be a lower oxide of the element, was in fact a different condition, an allotropic form, as it was then called, of pure phosphorus—a form which differs as widely from the wax-like, highly combustible material that is so well known as any two substances well could differ. A few years earlier Schönbein had discovered a new condition of oxygen, which he called ozone, differing widely from ordinary oxygen gas. Now, since all the forms of the same element yield the same products, and hence give the same chemical reactions, it became obvious, as such facts multiplied, that we may have different substances consisting wholly of the same chemical element; and hence that the chemical element, whatever it might be, could not be a definite substance, as Lavoisier had defined it.
Meanwhile another class of facts became prominent, chiefly in consequence of the investigations in organic chemistry to which Liebig had given such great impulse in Germany. Groups of compounds, consisting for the most part of carbon, hydrogen, and oxygen, came to be known, which, although having exactly the same composition (that is, formed by the union of the same elements in the same definite proportions), had, nevertheless, utterly different properties and relations. Such compounds are said to be isomeric, and a good example may be found in acetic ether, a very fragrant neutral spirit, and butyric acid, whose offensive odor and acrid taste are only too well known in rancid butter. But if oxygen is the acidifying principle of butyric acid, why does it not produce the same effect as an equal constituent of the ether? Similar phenomena of isomerism soon became very prominent, and forced on chemists the conviction, often against their prejudices, that the nature of the product depended not solely on the nature and proportions of the elements which entered into its composition, but quite as much, and even more, on the manner in which the constituents were combined.
To this phrase—the manner in which the constituents are combined—no definite meaning was at first attached; but the old atomic theory, first applied in chemistry by Dalton, was soon so modified as to give a form to the conception, and on the distinction between atoms and molecules then introduced the whole philosophy of modern chemistry rests.
In the subdivisions of material bodies, the molecules are the smallest masses in which the qualities of a substance inhere. A molecule of sugar or salt is simply a very small lump of sugar or salt in which all the qualities of sweetness or saltness are preserved. These molecules, however, although the elements of substances, are not the ultimate elements of matter, but on the contrary are themselves aggregates—often very complex aggregates—of still smaller units which are considered to be the elemental atoms. Of such atoms we must admit as many different kinds as we have distinguished chemical elements, and the atoms are for the present the ultimate limit of our analysis of matter. These atoms are now the ideal chemical elements. Starting from the atoms, the orders of combination are, first, the union of the atoms to form the molecules which are the nuclei of definite substances, then the aggregation of these molecules to form material masses.
Obviously we may conceive of the union of either similar or of dissimilar atoms; and while the union of unlike atoms results in the production of molecules of compound substances, the union of like atoms (all of oxygen or all of hydrogen, for example) yields molecules of elementary substances. So far as the primary structure is concerned, there is no distinction between an elementary substance like oxygen gas and a compound substance like water. In each case the material is an aggregate of similar molecules, and owes its physical qualities to the external relations of its peculiar units; but, while the molecules of oxygen gas are each composed of two atoms of oxygen, the molecules of water consist each of two atoms of hydrogen and one of oxygen. .
In admitting the possibility of the union of similar atoms to form the molecules of elementary substances, the new philosophy of chemistry differs most markedly from the old. The system of Lavoisier was based on a conception of dualism originally suggested by sexual relations; and the term elective affinity, which was so constantly used to explain chemical changes, was a phase of this conception. The elements of two kinds paired together to form acids or bases, and the acids and bases paired to form salts, and chemical changes were the consequence of the superior affinity of another acid or another base for the temporary mate of a fellow-companion. At the time of Lavoisier, the grosser features of these dualistic conceptions, which so disgust us in the earlier writers on chemistry, had disappeared; and, still later, Berzelius attempted to place the system on a scientific basis by referring the dualism to electrical relations. But there was an entire continuity of thought from first to last, and in this was involved the prevailing idea that strength of chemical union depended on opposition of qualities. But this idea, which I have no doubt many scholars who studied chemistry under the old system still retain, was an entire misconception.
One of the strongest combining forces known to chemistry is that which holds together the dissimilar atoms of oxygen and hydrogen in the molecules of water, and, measured by the heat evolved, this force is nearly equaled by the force which unites the similar atoms of nitrogen to form a molecule of nitrogen gas; and the great violence of many modern explosives depends upon this circumstance.
It will now be seen that with our new philosophy the whole glamour which formerly bedazzled our idea of an elementary substance, and distinguished it widely from all other substances, disappears. The differences between substances depend upon the differences between their molecules, and as great molecular differences may arise from the union of similar as from the union of dissimilar atoms. The union of two atoms of hydrogen and one of oxygen gives a molecule of water, the union of two atoms of hydrogen and two of oxygen gives a molecule of peroxide of hydrogen; the union of two atoms of oxygen alone gives a molecule of oxygen gas, the union of three atoms of oxygen a molecule of ozone, and the difference between the last two substances is as great and of the same sort as the difference between the first two; and so it is with the so-called allotropic states of other elementary substances.
According to the modern philosophy of chemistry, the properties and relations of a substance depend fully as much upon the manner in which the atoms are grouped in the molecules of the substance as upon the nature of the atoms of which the molecules consist; and the differences between isomeric substances to which we have referred, depend wholly on what we call the molecular structure. The molecules, both of butyric acid and of acetic ether, consist of four atoms of carbon, eight atoms of hydrogen, and two of oxygen, and the chemist will show you just how these atoms are grouped in the molecule of each substance, and how the separate relations of these widely differing products depend on the structure he has assigned to their respective molecules. Indeed, the study of molecular structure—that is, of the mode of grouping of atoms in the molecules, especially in those of the compounds of carbon—has almost engrossed the attention of chemists for the past twenty-five years. An immense mass of facts and theories has been collected, and a symbolical method of representing the structure has been adopted, which, although highly conventional, must embody real truth, however dimly it may be now perceived; for the system has led to more, and more important, discoveries than any one of the dominant systems of science of the present day. The system has a great charm for students, and what is called the study of organic chemistry in our colleges is wholly a discussion of problems of this kind.
These systems of atoms that we call molecules have been frequently compared to the solar system, and cited as evidence that man occupies an intermediate position in creation, with a microcosmos beneath, as far removed from the order of his perceptions as is the macrocosmos above him. To one who realizes what must be the complex dynamical relations as well as the order of magnitude of these molecular systems, the diagrams of molecular structure which may be seen in any work on organic chemistry can not but appear as crude and childish as the figures of constellations on a celestial globe; and when, as frequently happens, the student confounds the sign and the substance, one can hardly refrain from a little good-natured laugh at the spider-leg formulæ, as a noted German chemist is in the habit of calling them. Still, these are only the conventional forms of a good working theory, which is a noble product of human thought and an effective means of advancing knowledge.
For one who has followed the history of chemical thought from the first, it is easy to discover great imperfections in our present system. The assumption that, with more than seventy different kinds of atoms already known, uniting in such varied combinations to form molecules, only like molecules should ever aggregate to form material masses, is a solecism in the very postulates of the system; and the whole question of molecular combination is one which is still in abeyance. Analogy forbids us to believe that, down to a certain limit of dimensions that we call molecules, the constitution of matter is of a wholly different sort from that which appears on subdividing the molecules. It is an equally incredible assumption that all atoms of the same element are so many independent creations exactly alike in every respect. Then, as our knowledge increases, the distinctions between the chemical elements are becoming less marked and their relations to each other more intimate. They are beginning to appear, not as isolated units, but as links in a complex network, which presents an unbroken continuity throughout. The recent study of the rarer earths leaves us in doubt whether we have an indefinite number of elements, or only one under unnumbered manifestations; and the later results of spectrum analysis seem to indicate quite clearly that, at the high temperatures of the sun and of the fixed stars, many of our terrestrial elements are decomposed. From a mathematical analysis of the spectra, Grünwald maintains—and supports his conclusion by a great array of confirmatory measurements—that the remarkable solar spectrum line called helium, and the equally well-marked line of the sun's corona, come from two constituents of hydrogen gas, the first of which is somewhat heavier and the last far lighter than hydrogen gas; and this conclusion, if not finally accepted, is regarded as highly probable by men of such scientific sobriety as Liveing and Dewar, of Cambridge, in England—men who are acknowledged as among the best authorities on spectrum analysis. I had intended, in this connection, to discuss these last points, to which I can here only allude, and which are every day acquiring greater and greater importance; but my paper is already too long, and there is abundant material for another essay on the same general subject. I have accomplished the immediate object at which I aimed, if I have made evident that the foundations of our science are still hidden in obscurity, and that the conception of a chemical element is to-day just as indefinite and just as metaphysical as it was at the time of Aristotle.