Popular Science Monthly/Volume 8/February 1876/Are the Elements Elementary?
By F. W. CLARKE,
PROFESSOR OF CHEMISTRY AND PHYSICS IN THE UNIVERSITY OF CINCINNATI.
WHAT are the so-called chemical elements? Are they really elements, or only compounds of remarkable stability? It would be hard to find in physical science a question which has been oftener asked than this. It has furnished all sorts of investigators with abundant food for speculation. Men of the highest scientific ability have grappled with the problem, and left it still unsolved; others have constructed elaborate theories, which claimed to settle everything. Still the debate goes on. We cannot prove that the elements are truly what we call them, nor can we show beyond all doubt that they are compound in their nature. We may, however, weigh the opposing probabilities, and see which side of the question is the stronger. Whichever way the balance turns, the superstructure of chemistry will be but little affected. We know that all our nized compounds are formed by the union together of two or more supposed elements; and no revelations concerning the nature of the latter can well disturb that established knowledge. However we may speculate, the experimentally-ascertained facts will remain unaltered. They may receive slightly different theoretical interpretations, without having their practical bearings changed in the least degree.
The prevalent view of the subject, that the elements are elementary, is held by philosophical chemists in a purely provisional way. We need a convenient working hypothesis, and these sixty-three substances are elements for aught we absolutely know to the contrary. As far as we are at present experimentally concerned, then, we call them elements, bearing always in mind the possibility that they may be compounds. They have never been decomposed; we have no means adequate to their analysis; not one of them can be obtained from materials in which it does not already exist. But all this evidence is only negative. How do we know but that some future discovery may render possible the decomposition of these supposed elements? Shall we assert positively that we have reached the ultimate analysis, and may never hope to go any farther? Obviously, so definite a statement would be unjustifiable, and no sane chemist would venture to make it. The uncertainty of the subject may well be illustrated by a reference to chemical history. At the beginning of the present century the alkalies and alkaline earths were thought to be elements. They were not decomposable by any means then known, so that the supposition was perfectly fair. A very few years passed away, the galvanic battery was brought into use, and presently it was found that each of these bodies was a compound, containing a metal united with oxygen. Perhaps a similar advance in our knowledge may demonstrate the possibility of decomposing many of the substances now regarded as elementary. Such a discovery might work in either one of three ways. It might largely increase the number of supposed elements, by dividing each one into two or more new bodies. It might reduce the number by proving that our elements were formed by the union, in various proportions, of only a very few simpler substances, Or it might demonstrate the unity of matter, just as recent science has demonstrated the unity of force, and give us only one true element underlying all material forms. Such a culmination of our knowledge would be grand, indeed!
The evidence, then, upon which we assert the elementary nature of the fifty metals and thirteen non-metals, is very incomplete. On this side of the question there is really no other important testimony, save that just cited. Arguing from our present inability to decompose certain bodies, we assume for convenience that they are indecomposable. Now let us see what there is in favor of the opposite view.
One of the first things learned by the student in chemistry is, that the so-called elements are readily classifiable into a few natural groups. The members of any one of these groups resemble each other chemically in the closest manner, forming compounds of strong similarity, and often are very much alike in their physical properties also. The thought at once arises, Can these elements be totally distinct from each other—have they nothing in common—are these resemblances only due to chance? Such a supposition could scarcely be admitted, since Science excludes chance from her list of natural agencies. These relationships must mean something—but what?
If we look beyond the points of similarity to the points of difference between related elements, we shall find that these too are subject to regularity. The members of a group vary from each other, not in a meaningless, helter-skelter way, but systematically, so that they may be arranged in regular series. Take, for example, the group formed by the strikingly similar metals, calcium, strontium, and barium. If, now, we compare these with reference to any physical property, we shall find that strontium will always be between the other two. It is heavier than calcium and lighter than barium; and the same thing holds true of strontium compounds when compared with the corresponding compounds of its two associates. The integrity of the series is perfect; for in no case can the middle member be placed either at the beginning or the end. The nitrogen group is even more remarkable. Arranging its recognized members in the order of their atomic weight, they are as follows: nitrogen, phosphorus, arsenic, antimony, and bismuth. The first of these elements is gaseous at all known temperatures; phosphorus is a solid, but easily convertible into a gas by heat; arsenic is a denser body still, and less readily vaporized; antimony follows in regular order; and finally, bismuth, the heaviest of the series, can be distilled only with considerable difficulty. Here, then, is a gradation both in specific, gravity and in boiling-point, the lowest member of the group, in each of these particulars, being that with the lowest atomic weight; and the reverse. If we ascend from these elements to their compounds, we shall also notice some curious chemical regularities. Each member of the group unites with oxygen to form a pentoxide, from which an acid may be derived. Compare, now, these five acids: nitric is very strong, and violently corrosive; phosphoric is a little weaker, and acts much less vigorously; arsenic is feebler still; antimonic is extremely weak; and the corresponding bismuth compound is just barely recognizable as being an acid at all. Can these regular gradations be purely accidental and meaningless?
Examples like these might be adduced almost indefinitely. Series after series could be brought forward, all illustrating the same principle. Exceptions occur now and then, but they are so few that for present purposes they may be disregarded. Of course they mean something, but they are neither sufficiently abundant nor important enough to affect our arguments. The regularities are so numerous and so remarkable as to outweigh many times over all seeming variations. All this evidence is, however, inadequate in one respect: the relations thus far pointed out cannot be simply expressed in figures. Are there, then, any numerical relations connecting the elements? This question may be answered, partly by studying their atomic weights, and partly by an examination of their specific volumes.
The regularities which connect the elementary atomic weights have been examined and discussed by many investigators from widely differing points of view. Some chemists have contented themselves with the naked facts; others have considered the bearing of those facts upon chemical theories; and a third class, with less caution than ignorance, have speculated upon them in the wildest and most reckless manner. Of course a full summary of the whole subject, however interesting it might prove, would be out of place in a condensed argument like this. All we can do here is to glance at a few of the many relations known, and afterward consider them in their connection with our main subject. The general reader who cares to go deeper into the question will do well to consult the original papers of Dumas, Gladstone, J. P. Cooke, Kremers, Mendelejeff, and others.
Of the relations now under consideration, the one most frequently cited is as follows: Many elements are most naturally arranged in threes, of which the middle member has an atomic weight very nearly a mean between the atomic weights of the other two. Thus we have calcium, atomic weight, 40; strontium, 87.5; and barium, 137. Here, if the value of strontium were 88.5, it would be an exact mean. Again, chlorine has the atomic weight 35.5; bromine, 80; and iodine, 127; the second being almost precisely midway between the first and third. A still closer agreement with theory is furnished by lithium, sodium, and potassium, whose values are respectively 7, 23, and 39.1. A fourth example is afforded by potassium, 39.1; rubidium, 85.4; and cæsium, 133; while a fifth case is offered by phosphorus, 31; arsenic, 75; and antimony, 122. To be sure, these illustrations afford only an approximation to regularity; but then the variations are themselves somewhat regular. In each of these twos the middle term is just a little too low to be an absolute mean between its associates; that is, the variations from theory are all in one direction. It is hardly possible at present to say whether this means anything, or is only ascribable to accident. One more example of regularity among atomic weights is worth noting, namely, the relation which connects the members of the oxygen group. Here we have oxygen, 16; sulphur, 32; selenium, 79.5; and tellurium, 128. These higher numbers are simple multiples of the lowest; there being only a variation of half a unit (minus) in the case of selenium. Since these elements are very similar in their chemical relations, this regularity is extremely significant. Can it be due to chance, and void of real meaning?
But all these relations prove nothing—they merely suggest. Standing by themselves they would signify comparatively little; but considered with other analogous evidence they help to found an almost overwhelming argument. The concurrent testimony supplied by the specific or atomic volumes of the elements is particularly strong.
The specific volume of any substance is the quotient obtained upon dividing its atomic weight by its specific gravity. This value may be supposed to represent the volume of an atom of the substance plus the sphere of unoccupied space immediately surrounding and belonging to it. Leaving theoretical definitions out of account, however, we shall find, upon comparing the specific volumes of solid and liquid substances, many extraordinary relations. Often, all the members of an elementary group have equal values. This is the case with the closely-related metals platinum, iridium, osmium, palladium, rhodium, and ruthenium. They have different atomic weights and different specific gravities; yet the quotient obtained upon dividing the former by the latter is the same in every instance. The same thing holds good of the group formed by iron, cobalt, nickel, chromium, manganese, copper, and perhaps also uranium. Here the regularity extends even beyond the elements themselves, for their corresponding compounds have, with few exceptions, equal specific volumes also. An altogether different, but on the whole more remarkable, relation is furnished by the alkaline metals lithium, sodium, potassium, and rubidium; whose specific volumes are respectively 11.9, 23.7, 45.1, and 56.2. These values are almost exactly multiples of the first, standing to it in the ratio of 1 : 2 : 4 : 5. The slight variations from accuracy in this case are very far within the limits of experimental error. Almost as remarkable multiple relations are found in several other series, and apply not only to the specific volumes of the solid elements, but to their values in liquid compounds also. Closely connected with this subject is that of crystalline form. As a general, though not invariable rule, elements having equal specific volumes are isomorphous; that is, crystallize alike; a fact which may be extended to a very large number of compound series as well.
It would be easy to go on to almost an indefinite extent multiplying examples of relationship between the elements. There is hardly any set of physical properties which may not be made to emphasize the idea that these substances are internally related. Take, for example, their specific heats, which, multiplied by their atomic weights, give a constant quantity in the neighborhood of 6.3. That is, according to the law of Dulong and Petit, all elementary atoms have equal capacities for heat. But space is limited, so that we must omit the consideration of many important facts, and pass to the theoretical discussion of those already cited. All this evidence suggests quite emphatically that the elements are not totally distinct and independent bodies. Are they, then, compounds formed from a few simple substances, or are they modifications of but one primal matter? Strong arguments may be adduced in favor of either view, although neither can be yet demonstrated.
The idea that a very few true elements, uniting together in a variety of proportions, may give rise to all the bodies which we now look upon as elementary, derives perhaps its strongest support from an analogy pointed out by Prof. Cooke something like twenty years ago. He first called attention to the many serial relations which connect the members of any elementary group, and then showed how much these groups resemble the homologous series of organic chemistry. In such a series we have a number of compounds each differing from its immediate predecessor in a very definite way. Thus, in the series of alcohol radicles, we have first the hydrocarbon methyl. Adding to this an atom of carbon and two of hydrogen, we get the second member of the series; the third is formed by the same addition to the second, the fourth similarly derived from the third, and so on. The difference between the molecular weights of any two successive members in this series is always the same. Just so in some groups of elements, as we have already seen. The atomic weight of lithium is seven, add sixteen and we get that of sodium, while another increase of sixteen gives the value of potassium. Again, the atomic weight of sulphur is that of oxygen plus sixteen; three times sixteen more brings us to selenium, and another forty-eight reaches the equivalent of tellurium. Here certain multiples of sixteen are missing; do they correspond to the atomic weights of undiscovered elements? Such a speculation is curious, but not very profitable.
The analogy, then, between the groups of elements and the homologous series of organic compounds is quite striking, although it may not be very precise. Hence Cooke suggested that, if the elements were compounds, their resemblances might be explained by supposing them to form series like the hydrocarbons, in which bodies of similar constitution are akin in general properties. Now, this conception was certainly very brilliant, and rendered intelligible many important facts which before it were unclassified. It did not, however, suggest the possible unity of matter, but merely put the ultimate question regarding the nature of the elements a step farther back. Instead of many, it gave us the idea of few elementary bodies; why and how these differed were yet to be found out. Prof. Cooke was, fortunately, too cautious a chemist to put forward views of this sort dogmatically; he did not offer a theory even; he only made suggestions to be taken later at their true value, whatever that might be.
The other side of the question, that of the unity of matter, has been worked up by several chemists in a variety of ways. Some have studied the phenomena of crystallization and drawn their conclusions therefrom; others have taken up the subject from a dynamical point of view. Given atoms of one kind only, how to arrange these in different aggregations so as to present all the phenomena offered by our supposed elements in their relations to the various modes of energy? Perhaps in the discussion of this problem Gustavus Hinrichs would stand first. His conclusions may be easily questioned, but the ability and ingenuity displayed in reaching them cannot be denied.
To the general reader, or to the beginner in chemistry, the difficulties confronting the unitary view of matter may seem to be very great. Doubtless they are; but then every side of the subject is beset with difficulties. Obstacles must be surmounted, and the worst are not in this direction. The mind unused to speculations of this sort will probably encounter its greatest embarrassment in trying to understand how one substance alone can assume such a diversity of forms. That such a thing is within the limits of possibility, may be illustrated by reference to the facts of allotropy and isomerism. Quite a number of our present elements are known to be capable of existing in a variety of dissimilar modifications. Carbon is found as charcoal, graphite, and diamond; phosphorus exists both in its white and in its red modifications; oxygen is allotropic as ozone. Similar examples are furnished by arsenic, selenium, and, very notably, by sulphur. Among compounds, especially in organic chemistry, many cases occur in which several different bodies have precisely the same elementary composition. For instance, the essential oils of rose, bergamot, orange, lemon, lavender, turpentine, rosemary, nutmegs, myrtle, peppermint, etc., unlike as they may be in outward properties, are all composed of carbon and hydrogen in exactly the same percentages. The same atoms occur, but differently arranged. Many other sets of isomeric bodies are known in which this diversity of atomic arrangement can be distinctly traced, and the reasons for difference clearly pointed out. The limitations of space prevent their description here.
Now, since a single element may exist in several different forms, and since the same atoms can unite together so as to produce compounds very unlike each other, the chief objection to the unitary view is removed. Why may not all the so-called elements be allotropic modifications of one, or else isomeric bodies formed by the union of two or three such modifications? Such a supposition is by no means absurd, although, to be sure, it is not capable of rigid demonstration. It is only a speculation, but then within it are some fair probabilities. These may be strengthened by an appeal to spectroscopic evidence, and to the prevalent hypothesis concerning the origin of our planet.
If we examine the spectra of our supposed elements, we shall notice no more striking fact than the extent to which they differ in complexity. Some bodies give spectra of only one or two lines, while others are represented by hundreds. This atom emits light of a single wave-length, that one gives out rays of nearly half a thousand different kinds. Now, what do these facts mean? Do they indicate structural differences within molecules such that each bright line in a spectrum corresponds to a true element? Such a notion, if true, would lead to an alarming multiplication of elementary bodies, increasing our present confusion to an indefinite extent. If every possible wave-length of light represented a special element, the number of elements would be infinite. Clearly, then, this speculation, although frequently suggested, has very little to recommend it, and need not be entertained. Still, the fact of varying complexity among the elementary spectra remains to be accounted for. It certainly suggests a corresponding difference of complexity among the elements themselves, but of what nature? This question can hardly be answered directly, although it admits of interesting discussion, for which, unfortunately, we have little space to spare. Suffice it to say that spectroscopic phenomena are quite in harmony with the idea that all matter is at bottom one, our supposed atoms being really various aggregations of the same fundamental unit. It is approximately true that the simpler spectra are furnished by the elements of low atomic weight, while the multitudes of lines come from the heavier atoms. There are prominent exceptions to this rule, still it affords some support to our central idea.
But the spectroscope makes its most emphatic suggestions in favor of the unity of matter when it is applied to the study of the heavenly bodies. This subject I discussed in The Popular Science Monthly for January, 1873, and some months later Lockyer gave it prominence in England, his paper calling forth a good deal of comment. Therefore, only a brief résumé of my original suggestions is desirable now.
Everybody knows that the nebular hypothesis, as it is to-day, draws its strongest support from spectroscopic facts. There shine the nebulæ in the heavens, and the spectroscope tells us what they really are, namely, vast clouds of incandescent gas, mainly, if not entirely, hydrogen and nitrogen. If we attempt to trace the chain of evolution through which our planet is supposed to have grown, we shall find the sky is full of intermediate forms. The nebulæ themselves appear to be in various stages of development; the fixed stars or suns differ widely in chemical constitution and in temperature; our earth is most complex of all. There are no "missing links" such as the zoologist longs to discover when he tries to explain the origin of species. First, we have a nebula containing little more than hydrogen; then a very hot star with calcium, magnesium, and one or two other metals added; next comes a cooler sun in which free hydrogen is missing, but whose chemical complexity is much increased; at last we reach the true planets with their multitudes of material forms. Could there well be a more straightforward story? Could the unity of creation receive a much more ringing emphasis? We see the evolution of planets from nebulæ still going on, and parallel with it an evolution of higher from lower kinds of matter.
Just here, perhaps, is the key to the whole subject. If the elements are all in essence one, how could their many forms originate save by a process of evolution upward? How could their numerous relations with each other, and their regular serial arrangements into groups, be better explained? In this, as in other problems, the hypothesis of evolution is the simplest, most natural, and best in accordance with facts. Toward it all the lines of argument presented in this article converge. Atomic weights, specific volumes, and spectra, all unite in telling the same story, that our many elements have been derived from simpler stock.
I know that all this is only speculation, but surely it is not baseless. Science is constantly reaching forward from the known to the unknown, partly by careful experiment, and partly by the prophetic vision of thought. It first discovers facts, and then seeks to interpret them, although oftentimes the interpretation is not capable of absolute proof. So with the material of this article. We have seen that many relations connect in some mysterious way those bodies which we commonly regard as simple, and we have sought to determine their meaning. What can they mean, save that the elements are not elementary? How could the elements have originated but by a process of evolution?