Popular Science Monthly/Volume 18/February 1881/Evolution of the Chemical Elements
By LESTER F. WARD, A. M.
MUCH may be said in favor of the hypothesis of the progressive development of all the stable forms of matter by a true process of evolution from antecedent states. Indeed, in the higher forms of matter, in those which we know to be of composite constitution, this process is more or less thoroughly understood. Most of the objects which surround us, whether organic or inorganic, are known to consist of a great number of elementary parts of the same size and form which are aggregated in definite ways to form the general mass which each such object presents. These particles, which are alike for all parts of the same object or species of object, are unlike for different objects. Each object is an aggregate of elements of the same species, and these elements are the units of aggregation. All aggregates which have been thus far resolved into these units have confirmed this law. What is known, however, of the higher aggregates of matter is sufficient to establish another law, viz., that such aggregates are the result of the successive recompounding of units of aggregation of descending orders. The units of aggregation of aggregates of the higher orders are compounds of lower units. This is physically proved to be true of all aggregates of known composition.
In biology we have the individuals of various orders, both animal and vegetable, in which the lower forms are taken up bodily and made to enter as integral units into the higher forms. Not only are all animals and plants compounded of innumerable cells as ultimate biological units, but the earlier forms, which are aggregations of cells, are repeated as units in the higher forms. The tape-worm is an animal of the third order, the cell being taken as the first, but its segments are so feebly integrated that they possess all the essential characteristics of perfect animals. In the higher Annulosa, the integration is more complete, but the composite character is still evident. In the Vertebrata, the process of coördination has proceeded so far that only the closest embryological study can reveal their composite nature. On the other hand, corals as well as many protists, such as the Labyrinthuleæ, coexist with so small a degree of integration that the parts are considered as distinct individuals, although clearly dependent on one another.
The vegetable kingdom illustrates still more clearly the manner in which the aggregates are compounded. We have plants, like Caulerpa, which, while the form would lead us to expect a considerable degree of organization, consist in reality of a simple aggregation of homogeneous cells. In higher plants, the leaf forms a new order of organization, and constitutes the morphological individual or unit. In trees, the process of compounding has gone so far that, considered as individuals, they may reach the hundredth degree.
If we contemplate the mineral kingdom, we are again shown the same truth. The various recognized minerals are not generally found to be composed directly of the simple chemical compounds into which they may be resolved, but consist of compounds of different orders into which the simpler compounds enter as units of composition. Thus feldspar contains silica, alumina, peroxide of iron, lime, soda, potash, magnesia, water, etc., as units of composition, none of which is supposed to exist in the mineral in any simpler state, and all of which are already more or less complex chemical compounds. Moreover, two or more of these minerals thus formed often again combine as new units to form others of still higher organization.
When we consider the facts which chemistry furnishes, we see the same law still operating in great simplicity. In many of the binary, ternary, and higher compounds, theory requires us to assume that the substances entering into them do so in their integral state, and are not first decomposed into their primary elements and then reorganized into the new compound. The hydrated oxide of potassium, for example, is not written , but , in which both the immediate constituents are regarded as maintaining their composite state and entering bodily into the new compound. The entire series of "compound radicles" requires the same supposition and illustrates the same general principle. Cyanogen (), ammonium (), methyl (), ethyl (), and the rest are now held to constitute integral units in the formation of the hydrides, alcohols, and acids.
So far, then, as induction can be depended upon, we find that it is a universal law of the aggregation of matter that each new aggregate may become a unit for the formation of aggregates of higher orders. Does this law cease with the so-called chemical elements, or are these themselves the products of molecular aggregation?
Without discussing the old and apparently insolvable problem of the divisibility of matter, it may be remarked that while the known facts of science are entirely satisfied with the hypothesis of an ultimate, finite unit of matter, of which all perceptible objects are but aggregations, at the same time they do not conflict with such a modification of that hypothesis as assumes the actual magnitude of these units so far reduced as to be practically infinitesimal. They only declare—but this they do in the most emphatic manner—that this reduction must not be so far continued as to make the ultimate atom equal to zero, in the sense of absolute nullity.
On this view, which is by no means a new one, of the ultimate constitution of matter, the units of the so-called chemical elements, even of those having the smallest atomic weights, may themselves be of a relatively high order of aggregation or organization, below which many degrees may exist in which the molecules are too minute to form bodies which the senses can in any manner detect. The interstellar ether may be explained as constituting one of the highest of these degrees, yet not high enough to form matter such as to be visibly subject to the law of gravitation. The nebulæ present the evidence of the lowest form of such so-called "ponderable matter," and these may be supposed to be the result of a gradual development resulting from the successive recompounding of the molecular aggregates, until they finally acquire a certain influence over one another and tend to molar aggregation, forming the nebular masses. At the outset these aggregates may be supposed to be entirely homogeneous, consisting wholly of molecules of the same degree of aggregation, but they soon differentiate into several distinct kinds of matter. These are the gases which the spectroscope reveals in some of the nebulæ. They have molecules of low atomic weights and remain gaseous at all temperatures artificially producible. This process of evolution, which is the same which we have seen to go on in all the well-known forms of matter, would seem also to continue throughout the history of the nebulæ and the organization of resultant planetary systems, developing many additional forms of matter, likewise characterized by the increasing mass of their molecules.
What the properties of those molecular aggregates may be whose activities can not be revealed to sense, is of course unknown. Conjecture even as to the probable number of degrees of aggregation from the ultimate atom to the supposed atom of hydrogen would of course be idle. But that such forms exist far down upon this inaccessible plane, having definite shapes, sizes, and activities, we are strongly led to assume, both by the facts already stated and by others presently to be set forth.
Passing over these lower stages, therefore, whose study belongs to the future of human science, or to possible beings endowed with finer faculties, and which may be said to belong to the domain of transcendental chemistry, we finally arrive at a class of aggregates of great, stability, but which, though still so minute that they can only be perceived when accumulated into masses, have nevertheless been studied in their free state by means of the various phenomena to which they give rise, either in their natural condition, or, as is usually the case, under certain artificial conditions to which the ingenuity of man has learned to subject them. As these aggregates are the lowest which can be perceived, they have been denominated elements, and are by some supposed to constitute the ultimate units of matter. But, independently of certain direct evidence against this view, it is far more consistent with what is now known of matter, and with the laws of thought, to regard them as the first or lowest stages of aggregation whose activities are capable of appealing, either directly or indirectly, to our senses. It is really no more probable that the so-called elements are the lowest subdivisions of matter than that the remotest stars visible are actually at the confines of the universe.
That these elements are capable of manifesting themselves to sense is the sole reason of our recognizing their existence; and the history of their discovery, by which their number has been so greatly increased, shows that their modes of manifestation are often so subtile as to escape all but the most thorough methods of detection. Many of these elements now universally recognized remained for a long time wholly unsuspected, and these then belonged to the great class of unknown aggregates. This interesting chapter in the history of science should suffice to teach us that below the known of to-day there lies a wide belt of the knowable unknown, and that other and still lower orders of aggregates will doubtless yet be induced to reveal their existence.
A reason for regarding these elementary substances as ultimate units has been supposed to be found in their great stability, which causes them to behave as if such were the case.
While there is one possible exception to this in the case of oxygen and the peculiar phenomena of ozone and antozone, it is indeed true, so far as known, of all the remaining elements, that they have thus far resisted all attempts to decompose them. This, however, aside from the possibility of doing so still, is really no evidence of their absolutely elementary character, but only indicates what the whole theory of evolution would admit, if not require, that all aggregates which could possess the properties requisite for the composition of such masses as are capable of affecting the senses, or of so affecting other masses as to make themselves known to the human intellect, must possess a degree of inherent stability sufficient to resist all human efforts to disintegrate them. While, therefore, it is very probable that, just as the alkalies and alkaline earths, which, at the beginning of the present century, were regarded as elementary, have yielded to the galvanic battery and proved to be composite, so a few more of those now classed as elements will at no distant day be similarly decomposed by the higher appliances yet to be devised; it is nevertheless entirely consonant with the view of the constitution of matter here maintained, that there shall remain upon the plane of human investigation a greater or less number of wholly undecomposable aggregates, serving as the primary basis of all tangible substances.
It must be expected, however, that these elements will possess all degrees of capacity for manifesting their presence, and that while some will stand out boldly, cohere in vast masses, like iron, for example, and in various ways render themselves obvious and obtrusive, others will be ever hugging the confines of the imperceptible, and, like ozone, will perpetually evade the full scrutiny of science. To this latter class also belongs the substance which emits the green ray of the solar spectrum, which has already led eminent chemists to conjecture that it may be of simpler constitution than any recognized element, if not the primary form of matter.
Setting out with the elements, regarded as aggregates of a comparatively high order and stable organization, but differing from one another in form, size, and molecular activities, as widely as the masses they form differ in properties, the problem of the formation of the higher orders of aggregates becomes comparatively simple. We find ourselves already in the domain of experimental science where the more or less completely demonstrated laws of chemistry and molecular physics lead us up to the formation of the various inorganic and organic forms of matter. The constitution of the various substances found upon the earth is readily determined by decomposing them and weighing their constituents. The precise conditions, however, which have resulted in their formation as we find them and brought about the existing state of things in the universe, are not so easily determined, and for this purpose a further extension of the general law of material aggregation is required.
The study of the earth's crust clearly indicates that very different conditions have existed upon it in the remote past from those which we now find. The facts as a whole prove beyond a doubt that our globe has once been in a state both of greater or less liquidity and also of great heat, and that, as its surface has cooled down, the solid parts, to which alone we have access, have been formed, though to what depth these extend we are still ignorant. But, notwithstanding certain doubts which have from time to time been cast upon it, the theory which was very early advanced as most in harmony with the probable history of the planet, and according to which the cooling process has not yet reached the great interior, which is therefore still in a heated and molten, condition, still furnishes, perhaps, the most rational explanation yet made of the phenomena which the earth presents, and also best satisfies the a priori requirements.
It is now generally believed that the present condition of our earth, and also that of the entire solar system, has been the result of a cosmical process of development by which its matter, unchanged in quantity, has been slowly condensed from a diffused nebulous state, occupying enormously increased space—a condition analogous to, if not identical with, that which is now presented by a large number of irresolvable nebulæ whose spectra show them to be composed of gaseous matter in an incandescent state. This gaseous or nebulous condition, though exceedingly rare relatively to the solid forms of matter familiar to us, is nevertheless a state of a high degree of aggregation as compared with the forms of matter by which it is surrounded and with its wholly unaggregated state. Before the operations which may be designated as molar can commence, a degree of aggregation must be reached far exceeding that which exists in those molecules which are the vehicles of luminiferous radiations. The particles constituting the ethereal matter of interstellar space must be supposed to be so minute and relatively far separated as not to exert any appreciable influence upon one another tending to produce molar motion or organization; a condition which is explained on the same grounds as the fact that one system in space exerts no appreciable influence upon another system.
If the so-called chemical elements are simply so many stable molecular aggregates, whose differences are due to different modes and degrees of aggregation, then the gases of our earth are simply the most diffused state in which masses of these aggregates can be obtained. A gas is a diffused mass of homogeneous molecules, and this definition is as true of the compound gases, steam, carbonic acid, or vapor of alcohol, as it is of the simple ones, such as hydrogen, nitrogen, or vapor of mercury. It might, then, be naturally supposed that the nebulæ would contain a number of such gases, and as it is scarcely to be presumed that all the modes of forming stable aggregates are represented on our planet, so, in addition to some of those found here, it is reasonable to expect that nebulæ will contain some not known to us. In so far as the spectroscope—to which, indeed, we owe all our positive evidence of the existence of true nebulæ—is able to inform us, this view is confirmed. Two of our commonest gases, hydrogen and nitrogen, have been identified in nebulæ, while a third has been discovered which has not yet been identified with any known element.
Every modification of the nebular hypothesis yet put forth has been compelled to assume that the original nebulous mass must be in an incandescent state. Certain it is that all visible nebulæ are selfluminous. But this is a condition of their visibility. It can not be known how many may exist which have not yet reached this state, and are, therefore, invisible. It does not seem necessary to suppose that the contraction of a nebulous mass is either due to, or requires, a high temperature. No reason exists why cold particles may not become collected into a diffused mass. The inherent motions of these particles are not increased or diminished. But, these motions remaining the same, their circuits of motion are reduced, the frequency of contact is increased, and heat and light are evolved from the friction. The tendency of all matter under the law of gravitation, considered as an unexplained fact, is toward concentration. The evolution of heat is rather the check put upon this tendency, and, in so far as it exerts any influence, it exerts it in a direction the reverse of gravitation. There is a perpetual and rhythmic antagonism between the forces of integration and disintegration. When for any reason the former acquires an impetus which carries it to great lengths, it is resisted with increasing violence by the antithetical force evolving great heat, and eventually restoring the normal equilibrium. It seems altogether probable, therefore, that in the process of contraction of a nebulous mass, and its resolution into a system of worlds, the amount of heat radiated is in the end equal to the amount produced by condensation, which disposes entirely of the supposition that there must exist an incandescent nebula at the outset. The so-called "cooling off" is only apparent, and, while at times the amount of heat may be diminished, at other times it will be correspondingly increased. If the radiation of heat from the surface of a body into space tends to cool it off, so does the constant diminution of its volume without loss of mass tend to heat it, and throughout its career these two influences must antagonize each other. It is only after the limit to possible contraction, due to the nature of matter itself, begins to be reached that the amount of radiation of heat comes greatly to exceed the amount of its generation, and that the body actually begins to cool off.
During the greater part of the history of an evolving system, the central mass must possess an enormously high temperature. This is required by chemistry as well as by physics. Throughout nearly the whole of this period, all the matter of the system must exist in the form of gas. But there exist in our globe many substances whose existence in the gaseous state presupposes great heat. The degree of heat required to volatilize the metals is immense, and there are certain other substances, such as silicon, for which still greater temperatures are demanded. It would, however, be a violent assumption to suppose that the parent nebulæ, out of which the solar system was formed, contained from the outset in this diffused state all the substances which are found on the earth. It is much more reasonable, and our hypothesis permits us, to assume that these substances, requiring so great heat to liquefy and volatilize them, have been created, i. e., developed, during the progress of the formation of the system out of materials already existing in other forms and states of aggregation. On the supposition that during the earlier part, and perhaps during all but the very latest period, of this process the temperature of the nascent system was increasing, it is reasonable to assume that the intense heat would cause the breaking up of some of the molecular aggregates which were capable of maintaining the gaseous form at low temperatures, and would at the same time cause the formation of new aggregates only capable of maintaining that form under the high temperatures to which they were subjected at the time of their formation, many of which, nevertheless, would prove sufficiently stable to preserve the new form of aggregation after the temperature should go down, and, instead of reverting to their former condition on the cooling of the system, would assume successively the liquid and the solid states, and become constituent parts of and distinct substances in the cooled-off planets.
This theory of the origin of all those elementary terrestrial substances which require great heat to convert them into gas is supported by some facts. In the first place, none of the gases of these substances have been discovered in any of the nebulæ. The only two terrestrial substances, thus far determined with any certainty, are hydrogen and nitrogen. The latter of these exists in a free state in the earth's atmosphere, forming about four fifths of its volume and three fourths of its weight. The former does not exist in a free state in the atmosphere, in consequence of its strong affinity for oxygen, which is present there in excess, and whose union with it forms the waters of the globe. Both of these substances are gases at all temperatures producible by artificial means, and have only very recently been made to assume the liquid and solid states by the use of extraordinary devices. The other definite line which the spectrum of certain nebulæ presents is near to that of barium, but is conceded not to be the barium-line. It is, therefore, an unknown substance, and nothing can be said of its properties. Its proximity to the barium-line in the spectrum can not certainly be taken to indicate any special resemblance to that metal; and it is probably a gas at low temperatures, like hydrogen and nitrogen.
In the second place, as to these two last-named substances, one of them, hydrogen, is present in nearly or quite all the self-luminous bodies whose spectra have been observed, where it seems to occupy a position far out in the upper atmosphere. As to nitrogen, its presence in such bodies is doubtful, so far as the spectroscope is able to inform us; but, as it exists in such quantities in the earth's atmosphere, the belief is strong, especially among those who accept the nebular hypothesis, that the failure to discover it there is due to our imperfect methods, or to our ignorance of the manner in which the phenomena of the spectroscope are to be interpreted. The recent triumph of science, in the discovery of oxygen in the sun, serves to show how easy it is to overlook phenomena all the while perceptible, and gives great hope that not only nitrogen, but many other substances, will yet be found there, which have hitherto escaped observation. The fact that an element exists in the earth may not be proof that it must exist in the sun, even on the assumption that the sun is the parent of all the planets, but it is certainly strong presumptive evidence that it is also there. It is, however, much stronger proof that it existed in the general mass, as late at least as when the earth was formed out of it, and therefore in the original nebulæ. Those evolutionists alone who are ready to accept the view here advanced of the derivation of the heavier elements from the lighter ones, in the course of the development of the system, can escape this conclusion by supposing that the substance in question was created in the planet after its separation from the central mass. But this assumption would not be required in the case of nitrogen, which remains a gas at high temperatures, and which actually exists in the nebulæ. The fact that it can be detected in the nebulæ, and not in the sun, although it doubtless abounds in both, may be accounted for by remembering that the spectrum of a nebula belongs to a different class from that of the sun, the former consisting of bright lines on a dark ground, indicating a luminous gas; while the latter consists of dark lines on a bright ground, indicating a body having an incandescent solid or liquid interior, the rays of which pass through a cooler gaseous atmosphere. Now, this antithesis in the constitution of the two bodies may explain why certain elements existing in both may be capable of spectroscopic determination only in one, owing to peculiar conditions supplied by the special nature of the substances themselves; for it is by no means probable that the spectroscope gives us an account of all the substances existing in the bodies examined by it.
While, therefore, there is nothing in the facts thus far discovered which is opposed to the theory that the terrestrial substances having high melting and volatilizing points have been developed out of substances which are gaseous at lower temperatures in the course of the evolution of planetary systems, these facts, so far as they bear at all upon the problem, are decidedly favorable to such an hypothesis. We certainly find such substances in our earth and in the intensely heated bodies of space, as well as in such meteoric aggregates as from time to time reach our planet, and we have not yet found any such in existing nebulæ. If the latter be conceived as gaseous, and the solar system as only a developed state of one of them, either some such hypothesis must be brought forward to explain the existence of such substances in the earth, or the original mass must be supposed to possess a sufficient degree of heat to maintain them in a gaseous form, which would be enormous, and, independently of the present theory of the origin of the nebulæ, altogether improbable. Of course, upon the view here taken, it would be wholly inadmissible. Prior to the stage in the history of a nebula at which the degree of molar aggregation is sufficient to occasion a great amount of friction among the particles, the temperature of the primary molecular aggregates must be nearly that of space, and it can rise only as increase of density and molar motion increases that friction and converts material motion into ethereal vibration. Nebulæ must therefore possess a long history, of which neither the telescope nor the spectroscope can furnish any record—the pre-luminous period—in which, of course, no gases can exist except those, like hydrogen and nitrogen, which maintain their gaseous form under extremely low temperature. And it may be supposed that during this period other gases may exist associated with these, which, however, unlike them, are unable to sustain the successively higher and higher temperatures which the nebula acquires in its process of condensation and organization into a system, and at certain stages of this process are dissociated and resolved into aggregates of a different constitution, suited to these temperatures. Some of these latter new aggregates would naturally assume the liquid and solid forms at temperatures still high as compared with those to which we are accustomed, and constitute in the cooled-off crust of the planets the various metals and metalloids. In this manner we should have no difficulty in accounting for the existence of all the elements found on the earth, even if it were positively known that only the lighter gases were present in the parent nebulæ.
The recognized elementary substances, presenting so many different qualities, vary greatly in their so-called "atomic weights." This means simply that their molecules vary greatly in mass. The hydrogen molecule is the least known, and is therefore taken as the standard. Compared with this as unity, we find that the molecule of oxygen contains 16 times as much matter, that of carbon contains 12, that of nitrogen 14, and that of chlorine 351 times as much. But these, instead of representing large equivalents, are, when compared with most of the metals, very small. One molecule of mercury contains two hundred times as much matter as one of hydrogen. The atomic weight of gold is 197, of platinum 197⋅4, of lead 207, and of bismuth 208; while the thorium equivalent, which was quadrupled in the new system, is now put at 231⋅4, being the largest of all the elementary units. Whether hydrogen, carbon, nitrogen, oxygen, or any of the other abundant elements having small molecules have entered into the composition of these heavy substances, is a legitimate question. The fact that these molecules are stable, whether combined or uncombined, is favorable to this view, although there may exist, as component units of the molecules of the metals, many equally stable aggregates which no human power can dissociate from their present combinations. But, if known elements were employed as components of other known elements having larger molecules, the very fact that they are elements, i. e., that we are unable to decompose them, would render it impossible to know that such was the case. Think how many hydrogen, nitrogen, or oxygen molecules might enter into the system that constitutes the unit of bismuth or of gold!
Now, it is a remarkable fact that those elements which have very high condensing points, i. e., which assume the liquid (or solid) form at very high temperatures, generally have large combining numbers, that is, large molecules; while those having low condensing points and which are gaseous at ordinary temperatures, as a rule have small combining numbers, or small molecules. To this, carbon on the one hand, and mercury on the other, form, it is true, notable exceptions; nevertheless, the bulk of the facts sustain this law, and indicate that the genesis of those elements which we know only as solids or liquids, and which we have supposed to have taken place during the fiery ordeal through which the solar system has had to pass, is rather a process of integration than of subdivision, since they have much larger molecules than the gases that exist in the nebulæ, and out of which we have supposed them to be formed.
We have seen that matter, in its cosmical history, as enacted in the development of a planetary system, assumes a great variety of forms, and resolves itself into numerous specifically distinct molecular aggregates. The different substances which we know on our planet are the result of the cohesion into homogeneous masses of these different aggregates, all the constituent units of any one of these masses consisting of the same species of molecular aggregate. We saw reason to suppose that, at an early period in the development of the solar system (and we may infer the same for all systems), the number of distinct substances was small, and that these substances were gaseous at very low temperatures. The two abundant gases, nitrogen and hydrogen, exist in the irresolvable gaseous nebulæ, and these, doubtless, went far to constitute the original substance of our infant system. These gases, though differing greatly from each other in their atomic weights, nevertheless have small molecules compared with those of most substances now found in the earth, and which are, for the most part, either solid or liquid at life-supporting temperature. There has, therefore, been upon the whole increase of mass among the molecules of substances later developed.
When we rise to the point of view which removes all distinction between elements and compounds, except the subjective one that in the former we do not know and cannot prove their composition, while in the latter we can do this in so far as to resolve them into the former, we can make the further generalization that along with this increase of mass there has gone decrease of stability in such molecules. We are of course unable to predicate this, except inferentially, of the elements which we can not decompose, although these, doubtless, vary greatly in their relative stability, and, as before remarked, some substances which had been supposed to be elementary have already been reduced to simpler forms, and others may still be so reduced. Moreover, those which have thus yielded possess large molecules (counting that of the compound), and this should serve as an index to future attempts of a like nature. There is, for example, little hope of resolving hydrogen or carbon into simpler elements, but the reverse of the alchemist's dream may yet be realized, and gold reduced, if not to baser, at least to simpler materials.
All the known chemical compounds must be supposed to have been developed within relatively quite recent periods. The great heat that has prevailed throughout the greater part of the history of the solar system, and which, indeed, still prevails in its nucleus, the sun, which is still 9913 (99⋅866) per cent, of the entire mass of the system, or practically the whole of it, has prevented the formation of any of the substances which we know to be composite. It is only in the comparatively minute masses which have been accidentally separated from the rest, and which, in consequence of their diminutive size, have earlier reached the point at which the radiation exceeds the generation of heat, that conditions have been produced under which these comparatively unstable substances, such as water, carbonic dioxide, and the other oxides comprising the earth's crust, could exist. In proportion as the degree of heat diminished, the capacity for more and more unstable substances increased. The earliest compounds were those in which silicon, potassium, sodium, magnesium, etc., combine with oxygen, several of which were, from their great stability, long regarded as elementary. Then came a variety of acids, alkalies, and salts, together with compounds of the metals. Later, as the temperature still further lowered, the oxygen was enabled to seize the hydrogen and form the gaseous protoxide, steam, which at a still later period, when the temperature of the earth's surface fell below 100° Centigrade, condensed into water. Long prior to this, carbonic acid had been formed, and, doubtless, constituted at that time fully one half of the earth's atmosphere. The vast amount of free carbon now existing in the earth, and, still more, that which is fixed in the chalk and limestone formations, all of which must have formerly existed in the atmosphere in the form of carbonic-acid gas, indicates that the above estimate is probably far too low.
All the compounds thus far referred to, and all others having a certain degree of stability, must have been first formed at a period of considerable heat, the dissociation point of all compounds having been estimated at 6,000° Centigrade; although this, doubtless, varies for different compounds as greatly as do the condensing points of different gases. But there are, besides, many compounds which are continually forming at such temperatures as now prevail on the surface of the earth, and most of these are very much more unstable than those last mentioned. The elements which chiefly enter into such compounds are oxygen, nitrogen, hydrogen, and carbon, all but the last named of which are gaseous at ordinary temperatures. The substances of this nature with which we are familiar are known as organic compounds, and such as we see are, in fact, the products of organized beings from the different parts of which they are obtained. But this process should not be regarded as any less cosmical than that by which the rocks or the metals have been evolved out of primordial matter.
Time forbids the further following out of this series of steps in the development of existing forms of matter, but it will be readily perceived how, if this train of reasoning be sound, the inorganic is directly linked to the organic world, just as the imperceptible forms of matter were shown to be linked to its perceptible forms, and the elementary states to the composite states, in one continuous and unbroken chain. It remains to point out to what extent the hypothesis here advanced, of the probable genesis of the chemical elements, is found to be in harmony with the recent discoveries which Mr. J. Norman Lockyer has made by means of the spectroscope in the domain of chemistry. In endeavoring to do this in the briefest manner possible, let us reproduce the following diagram drawn up by him:
Modified in the arrangement only to suit the present discussion, the first part of this diagram may be presented as follows:
The figures placed over the symbols are the respective atomic weights of the elements according to the new system, those placed beneath being the same according to the old system. Transposing calcium and magnesium merely for the sake of symmetry, their position being indifferent, since both appear in the hottest stars, we find that with a single exception, that of sodium, if we take the new system, and without exception, if we take the old system, the atomic weights increase as the temperature of the body diminishes. To what extent this result may be accidental it is of course impossible to say, but, so far as it may have any scientific significance, it constitutes an interesting confirmation of the theory that the heavier elements with large molecules have been developed out of the lighter ones with small molecules during the progress of the condensation and refrigeration of the heavenly bodies, and according to which, as above pointed out, those possessing the largest equivalents would be last formed and gradually pass into the known compounds by a corresponding gradual decrease of stability, these latter to be succeeded in turn by the evolution of organic aggregates which ushered in the era of life.
Generalizing from all that has been said, we may divide the known forms of matter into the three following classes, with the accompanying definitions:
1. Chemical Elements.—Substances whose molecules are composed either of those of other chemical elements of less atomic weight, or of such as are too low to be capable of molar aggregation, and therefore imperceptible to sense: formed during the progress of development of star-systems at temperatures higher than can be artificially produced, and hence too stable to be artificially dissociated.
2. Inorganic Compounds.—Substances whose molecules are composed of those of chemical elements or of other inorganic compounds of lower degrees of aggregation: formed in the later stages of the development of planets at high but artificially producible temperatures, and therefore capable of artificial decomposition; and constituting the greater part of the solid crust of cooled-off bodies, their liquid and a portion of their gaseous envelope.
3. Organic Compounds.—Substances whose highly complex and very unstable molecules are composed of those of chemical elements, inorganic compounds, or organic compounds of lower organization: formed on the cooled surfaces of fully developed planets at life-supporting temperatures.