A PHYSICAL THEORY OF THE UNIVERSE. From the North American Review, July, 1864. ³

In 1811 Sir William Herschel communicated to the Royal Society a paper in which he gave an exposition of his famous hypothesis of the transformation of nebulae into stars. “Assuming a self-luminous substance of a highly attenuated nature to be distributed through the celestial regions, he endeavored to show that, by the mutual attraction of its constituent parts, it would have a tendency to form itself into distinct aggregations of nebulous matter, which in each case would gradually condense from the continued action of the attractive forces, until the resulting mass finally acquired the consistency of a solid body, and became a star. In those instances wherein the collection of nebulous matter was very extensive, subordinate centres of attraction could not fail to be established, around which the adjacent particles would arrange themselves; and thus the whole mass would in process of time be transformed into a determinate number of discrete bodies, which would ultimately assume the condition of a cluster of stars. Herschel pointed out various circumstances which appeared to him to afford just grounds for believing that such a nebulous substance existed independently in space. He maintained that the phenomena of nebulous stars, and the changes observable in the great nebula of Orion, could not be satisfactorily accounted for by any other hypothesis. Admitting, then, the existence of a nebulous substance, he concluded, from indications of milky nebulosity which he encountered in the course of his observations, that it was distributed in great abundance throughout the celestial regions. The vast collections of nebulæ

which he had observed, of every variety of structure and in every stage of condensation, were employed by him with admirable address in illustrating the modus operandi of his hypothesis.” Grant’s History of Physical Astronomy. ⁴

Laplace, in his Système du Monde, applied this hypothesis, by an ingenious but simple use of mechanical principles, to the explanation of the origin of the planetary bodies, and of the general features of their movements in the solar system. Supposing the original nebulous mass to receive a rotatory motion by its aggregation, he showed that this motion would be quickened by a further contraction of the mass, until the centrifugal force of its equatorial regions would be sufficient to balance their gravitation, and to suspend them in the form of a vaporous ring. Again, supposing this revolving ring to be broken, and finally collected by a further aggregation into a spherical nebulous mass, he showed, in the same way, how the body of a planet, with its system of satellites, might be formed. The material and the original motions of the planets and their satellites could thus, he supposed, be successively produced, as the nebula gradually contracted to the dimensions of the sun.

No scientific theory has received a fairer treatment than the nebular hypothesis. Arising as it did as a speculative conclusion from one of the grandest inductions in the whole range of physical inquiry,—connecting as it does so many facts, though vaguely and inconclusively, into one system,—it possesses, what is rare in so bold and heterodox a view, a verisimilitude quite disproportionate to the real evidence which can be adduced in its support. The difficulties which ordinarily attend the reception of new ideas, were in this case removed beforehand. The hypothesis violated no habitual association of ideas, at least among those who were at all competent to comprehend its import. Though resting on a much feebler support of direct evidence than the astronomical theories of Copernicus, Galileo, and Kepler, it met with a cordial reception from its apparent accordance with certain preconceptions, of the same kind as those, which, though extrinsic and irrelevant

to scientific inquiry, were able to oppose themselves successfully for a long time to the ascertained truths of modern astronomy.

The test of conceivableness, the receptivity of the imagination, is a condition, if not of truth itself, at least of belief in the truth; and in this respect the nebular hypothesis was well founded. It belonged to that class of theories of which it is sometimes said, “that, if they are not true, they deserve to be true.” A place was already prepared for it in the imaginations and the speculative interests of the scientific world.

We propose to review briefly some of the conditions which have given so great a plausibility to this hypothesis. In the first place, on purely speculative grounds, this hypothesis, as a cosmological theory, happily combines the excellences of the two principal doctrines on the origin of the world that were held by the ancients, and which modern theorists have discussed as views which, though neither can be established scientifically, have no less interest from a theological point of view;—namely, first, the materialistic doctrine, that the world, though finite in the duration of its orderly successions and changes, is infinite in the duration of its material substance; and, secondly, the spiritualistic doctrine, that matter and form are equally the effects, finite in duration, of a spiritual and eternal cause.

At first sight the nebular hypothesis seems to agree most nearly with the materialistic cosmology, as taught by the greater number of the ancient philosophers; but the resemblance is only superficial, and, though the hypothesis possesses those qualities by which the ancient doctrine was suited to the limitations and requirements of the poetical imagination, yet it does not involve that element of fortuitous causation which gave to the ancient doctrine its atheistic character. In the nebular hypothesis the act of creation, though reduced to its simplest form, is still essentially the same as that which a spiritualistic cosmology requires. The first created matter filling the universe is devoid only of outward and developed forms, but contains created within it the forces which shall determine every change and circumstance of its subsequent history.

The hypothesis being thus at once simple and theistic appeals to imagination and feeling as one which at least ought to be true.

Such considerations as these doubtless determined the fate of another ancient cosmological doctrine, which, though adopted by Aristotle, was regarded with little favor by ancient philosophers generally. For there could be but little support, either from poetry or religion, to the doctrine which denied creation, and held that the order of nature is not, in its cosmical relations, a progression toward an end, or a development, but is rather an endless succession of changes, simple and constant in their elements, though infinite in their combinations, which constitute an order without beginning and without termination.

While this latter doctrine was not necessarily materialistic, like that which has been so termed, and which was more generally received among the ancients, and though it has the greater scientific simplicity, yet it fails on a point of prime importance, so far as its general acceptance is concerned, in that it ignores the main interest which commonly attaches to the problem. Cosmological speculations are, indeed, properly concerned with the mode or order of the creation, and not with the fact of the creation itself. But that the first cosmogonies were written in verse shows the almost dramatic interest which their themes inspired. “In the beginning” has never ceased to charm the imagination; and these are almost the only words in our own sacred cosmogony to which the modern geologist has not been compelled to give some ingenious interpretation. That there was a beginning of the order of natural events and successions may be said to be the almost universal faith of Christendom.

The nebular hypothesis, conforming to this preconception and to the greatest poetic simplicity, passed the ordeal of unscientific criticism with remarkable success. Not less was its success under a general scientific review. A large number of facts and relations, otherwise unaccounted for, become explicable as at least very probable consequences of its assumptions; and these assumptions were not, at first, without that independent

probability which a true scientific theory requires. The existence of the so-called nebulous matter was rendered very probable by the earlier revelations of the telescope; and, though subsequent researches in stellar astronomy have rather diminished than increased the antecedent probability of the theory, by successively resolving the nebulæ into clusters of star-like constituents,—suggesting that all nebulosity may arise from deficiency in the optical powers of the astronomer rather than inhere in the constitution of the nebulas themselves,—and thereby invalidating the scientific completeness of the theory, yet the plausible explanations which it still affords of the constitution of the solar system have saved it from condemnation with a considerable number of ingenious thinkers. With astronomers generally, however, it has gradually fallen in esteem. It retains too much of its original character of a happy guess, and has received too little confirmation of a precise and definite kind, to entitle it to rank highly as a physical theory.

But there are two principal grounds on which it will doubtless retain its claim to credibility, till its place is supplied, if this ever happens, by some more satisfactory account of cosmical phenomena. To one of these grounds we have just alluded. The details of the constitution of the solar system present, as we have said, many features which suggest a physical origin, directing inquiry as to how they were produced, rather than as to why they exist,—an inquiry into physical, rather than final causes; features of the same mixed character of regularity and apparent accident which are seen in the details of geological or biological phenomena; features not sufficiently regular to indicate a simple primary law, either physical or teleological, nor yet sufficiently irregular to show an absence of law and relation in their production.

The approximation of the orbits of the planets to a common plane, the common direction of their motions around the sun, the approximation of the planes and the directions of their rotations to the planes of their orbits and the directions of their revolutions, the approximatively regular distribution of their distances from the sun, the relations of their satellites to the general features of the primary system,—these are some

of the facts requiring explanations of the kind which a geologist or a naturalist would give of the distribution of minerals, or stratifications in the crust of the earth, or of the distribution of plants and animals upon its surface,—phenomena indicating complex antecedent conditions, in which the evidence of law is more or less distinct. The absence of that perfection in the solar system, of that unblemished completeness, which the ancient astronomy assumed and taught, and the presence, at the same time, of an apparently imperfect regularity, compel us to regard the constitution of the solar system as a secondary and derived product of complicated operations, instead of an archetypal and pure creation.

Such is one of the grounds on which the nebular hypothesis rests. The other is of a more general character. The antecedent probability which the theory lacks, from its inability to prove by independent evidence the fundamental assumption of a nebulous matter, is partially supplied by a still more general hypothesis, to which this theory may be regarded as in some sort a corollary. We refer to the “development hypothesis, or “theory of evolution,”—a generalization from certain biological phenomena, which has latterly attracted great attention from speculative naturalists. This hypothesis has been less fortunate in its history than that of the astronomical one. Inveterate prejudices, insoluble associations of ideas, a want of preparation in the habits of the imagination, were the unscientific obstacles to a general and ready acceptance of this hypothesis at its first promulgation. Though in one of its applications it is identical with the nebular hypothesis, yet, in more direct application to the phenomena of the general life on the earth’s surface, it appears so improbable, that it has hitherto failed to gain the favor which the nebular hypothesis enjoys. Nevertheless, as a general conception, and independently of its specific use in scientific theories, it has much to recommend it to the speculative mind. It is, as it were, an abstract statement of the order which the intellect expects to find in the phenomena of nature. “Evolution,” or the progress “from the homogeneous to the heterogeneous, and from the simple to the complex,” is the order of the progress

of knowledge itself, and is, therefore, naturally enough, sought for as the order in time of all natural phenomena. The specific natural phenomena in which the law of “evolution” is determined by observation as a real and established law, are the phenomena of the growth of the individual organism, animal or plant. As a law of psychological phenomena, and even of certain elements of social and historical phenomena, it is also well established. Its extension to the phenomena of the life of the races of organized beings, and to the successions of life on the surface of the earth, is still a speculative conclusion, with about the same degree of scientific probability that the nebular hypothesis possesses. And lastly, in the form of the nebular hypothesis itself, it is extended so as to include the whole series of the phenomena of the universe, and is thus in generality, if accepted as a law of nature, superior to any other generalization in the history of philosophy.

As included in this grander generalization, the nebular hypothesis receives a very important accession of probability, provided that this generalization can be regarded as otherwise well founded. As a part of the induction by which this generalization must be established, if it be capable of proof, the nebular hypothesis acquires a new and important interest.

We are far from being convinced, however, that further inquiry will succeed in establishing so interesting a conclusion. We strongly suspect that the law of “evolution” will fail to appear in phenomena not connected, either directly or remotely, with the life of the individual organism, of the growth of which this law is an abstract description. And, heterodox though the opinion be, we are inclined to accept as the soundest and most catholic assumption, on grounds of scientific method, the too little regarded doctrine of Aristotle, which banishes cosmology from the realm of scientific inquiry, reducing natural phenomena in their cosmical relations to ah infinite variety of manifestations (without a discoverable tendency on the whole) of causes and laws which are simple and constant in their ultimate elements. The laws or archetypes of nature are properly the laws of invariable or unconditional frequence in natural operations. And it is only with the objective relations of these laws, as constituting the order of nature, that natural science is concerned. Their subjective relations, origin, and essential being belong to the province of transcendental metaphysics, and to a philosophy of faith. According to this division, there can never arise any conflict between science and faith; for what the one is competent to declare, the other is incompetent to dispute. Science should be free to determine what the order of nature is, and faith equally free to declare the essential nature of causation or creation. ⁵

In rejecting the essential doctrine of “the theory of evolution” or “the development hypothesis,” we must reserve an important conclusion implied in the doctrine, which we think is its strongest point. There are several large classes of facts, apparently ultimate and unaccountable, which still bear the marks of being the consequences of the operations of so-called secondary causes,—in other words, have the same general character as phenomena which are known to be the results of mixed and conflicting causes, or exhibit at the same time evidence of law and appearance of accident. That such facts should be regarded as evidence of natural operations still unknown, and perhaps unsuspected, is, we think, a legitimate conclusion, and one which is presupposed in “the theory of evolution,” and in the nebular hypothesis, but does not necessitate the characteristic assumptions of these speculations. An extension of the sphere of secondary causes, even to the explanation of all the forms of the universe as it now exists, or of all the forms which we may conceive ever to have existed, is a very different thing from adopting the cosmological doctrine of the “development theory.” Naturalists who have recently become convinced of the necessity of extending natural explanations to facts in biology hitherto regarded as ultimate and inexplicable, but who are unwilling to adopt the cosmological view implied in the “development theory,” have adopted a new name to designate their views. “The derivative theory,” or “derivative hypothesis,” implies only continuity, not growth or progress, in the succession of races on the surface of the earth. Progress may have been made, as a matter of fact, and the evidence of it may be very conclusive in the geological record; but the fact may still be of secondary importance in the cosmological relations of the phenomena, and the theory ought not, therefore, to give the fact too prominent a place in its nomenclature.

That the constitution of the solar system is not archetypal, as the ancients supposed, but the same corrupt mixture of law and apparent accident that the phenomena of the earth’s surface exhibit, is evidence enough that this system is a natural product; This argument for physical causes is apparently the reverse of that which Laplace derived from the regularities of the solar system and the theory of probabilities; but in reality the objects of the two arguments are distinct. For the legitimate conclusion from Laplace’s computation is, not that the solar system is simply a physical product, but that the causes of its production could not have been irregular. The result of this computation was a probability of two hundred thousand billions to one that the regularities of the solar system are not the effects of chance or irregular causes. The gist of this argument is to prove simplicity in the antecedents of the solar system; and, had the proportion been still greater, or infinity to one, the argument might have proved a primitive or archetypal character in the movements of this system. It is therefore in the limitations, and not in the magnitude, of this proportion, that there is any tendency to show physical antecedence. Hence it is not from the regularities of the solar system, but from its complexity, that its physical origin is justly inferred. Regarding the law of causation as universal, since, if not implied in the very search for causes, it is at least the broadest and the best established induction from natural phenomena, we conclude that the appearance of accident among the manifestations of law is proof of the existence of complex antecedent conditions and of physical causation, and that the absence of this appearance is proof of simple and primitive law. ⁶ and the nebular hypothesis, so far as it is concerned with the explanation simply of the production of this system, and independently of its cosmological import, may be regarded as a legitimate theory, even on the ground we have assumed, though on this ground the most probable hypothesis would assimilate the causes which produced the solar system more nearly to the character of ordinary natural operations than the nebular hypothesis does. With a view to such assimilation, and in opposition to “the theory of evolution” as a generalization from the phenomena of growth, we will now propose another generalization, which we cannot but regard as better founded in the laws of nature. We may call it the principle of counter-movements,—a principle in accordance with which there is no action in nature to which there is not some counteraction, and no production in nature from which in infinite ages there can result an infinite product. In biological phenomena this principle is familiarly illustrated by the counter-play of the forces of life and death, of nutrition and waste, of growth and degeneration, and of similar opposite effects. In geology the movements of the materials of the earth’s crust through the counteractions of the forces by which the strata are elevated

and denuded, depressed and deposited, ground to mud or hardened to rock, are all of the compensative sort; and the movements of the gaseous and liquid oceans which surround the earth manifest still more markedly the principle of counter- -movements in the familiar phenomena of the weather.

Of what we may call cosmical weather, in the interstellar spaces, little is known. Of the general cosmical effects of the opposing actions of heat and gravitation, the great dispersive and concentrative principles of the universe, we can at present only form vague conjectures; but that these two principles are the agents of vast counter-movements in the formation and destruction of systems of worlds, always operative in neverending cycles and in infinite time, seems to us to be by far the most rational supposition which we can form concerning the matter. And indeed, in one form or another, the agencies of heat and gravitation must furnish the explanations of the circumstances and the peculiarities of solar and sidereal systems. These are the agents which the nebular hypothesis supposes; but by this hypothesis they are supposed to act under conditions opposed to that general analogy of natural operations expressed by the law of counter-movements. Their relative actions are regarded as directed, under certain conditions, toward a certain definite result; and this being attained, their formative agency is supposed to cease, the system to be finished, and the creation, though a continuous process, to be a limited one.

It should be noticed, however, in favor of the nebular hypothesis, that its assumptions are made, not arbitrarily, in opposition to the general analogy of natural operations, but because they furnish at once and very simply certain mechanical conditions from which systems analogous to the solar system may be shown to be derivable. The dispersive agency of heat is supposed to furnish the primordial conditions, upon which, as the heat is gradually lost from the clouds of nebulous matter, the agency of gravitation produces the condensations, the motions, and the disruptions of the masses which subsequently become suns and planets and satellites. And if the mechanical conditions assumed in this hypothesis could be

shown to be the only ones by which similar effects could be produced, the hypothesis would, without doubt, acquire a degree of probability amounting almost to certainty, even in spite of the absence of independent proof that matter has ever existed in the nebulous form.

But the mechanical conditions of the problem have never been determined in this exhaustive manner, nor are the conditions assumed in the nebular hypothesis able to determine any other than the general circumstances of the solar system, such as it is supposed to have in common with similar systems among the stars. A more detailed deduction would probably require as many separate, arbitrary, and additional hypotheses as there are special circumstances to be accounted for. Until, therefore, it can be shown that the nebular hypothesis is the only one which can account mechanically for the agency of heat and gravitation in the formation of special systems of worlds, like the solar system, its special cosmological and mechanical features ought to be regarded with suspicions, as opposed to the general analogy of natural operations.

We propose to criticise this hypothesis more in detail, and to indicate briefly the direction in which we believe a better solution of the problem of the construction of the solar system will be found. But before proceeding, we must notice an able Essay, by Mr. Herbert Spencer, the first in his Second Series of “Essays: Scientific, Political, and Speculative.”

In this essay on the “Nebular Hypothesis,” and in the following one on “Illogical Geology,” Mr. Spencer has attempted the beginning of that inductive proof of the general theory of “evolution” to which we have referred. Undoubtedly the clearest and the ablest of the champions and expounders of this theory, he brings to its illustration and defense an extraordinary sagacity, and an aptitude for dealing with scientific facts at second hand, and in their broad general relations, such as few discoverers and adepts in natural science have ever exhibited. For dealing with facts which are matters of common observation, his powers are those of true genius. In the essays following those with which we are immediately interested, and particularly in the essay on “The Physiology of Laughter,”

and in the review of Mr. Bain’s work on “The Emotions and the Will,” he displays the true scope of his genius. In psychology, and in the physiology of familiar facts, we regard his contributions to philosophy as of real and lasting value. He is deficient, however, in that technical knowledge which is necessary to a correct apprehension of the obscure facts of science; and his generalizations upon them do not impress us as so well founded as they are ingenious.

In his résumé of the facts favorable to the nebular hypothesis, he has committed sundry errors of minor importance, which do not in themselves materially affect the credibility of the hypothesis, but illustrate the extremely loose and uncertain character of the general arguments in its support. A singular use is made of a table, compiled by Arago, of the inclinations of the planes of the orbits of the comets. The legitimate inference from this table is, that there is a wellmarked accumulation of the planes of these orbits at small inclinations to the plane of the ecliptic. In considering the directions of the poles of these planes, we ought to find them equally distributed to all parts of the heavens, in case the orbits of the comets bear no relation to those of the planets or to each other. Instead of this, we find a marked concentration of these poles about the pole of the ecliptic, showing that their planes tend decidedly to coincide with the ecliptic. But Mr. Spencer has drawn from this table a conclusion directly the reverse of this. Assuming, as we cannot but believe on insufficient evidence, that the directions of the major axes of the orbits of those comets whose planes are greatly inclined to the ecliptic have nearly as great an inclination as they can have, or that they are nearly as much inclined to the ecliptic as the planes of the orbits themselves, he regards the table of the inclinations of the planes of the orbits as indicating, at least for such comets, the directions of their axes, and draws thence the conclusion, that there is a well-marked concentration about the pole of the directions of the axes of the cometary orbits, and hence, that the regions in which the aphelia of comets are most numerous are above and below the sun, in directions nearly perpendicular to the ecliptic. This

conclusion, though the reverse of that which is legitimately drawn from Arago’s table, is not inconsistent with it; and if Mr. Spencer were correct in his assumption concerning the directions of the axes of highly inclined orbits, the table would show that there are really two well-distinguished systems of comets, the one belonging to the general planetary system, and the other, Mr. Spencer’s, forming a system by itself,—an axial one, at right angles with the general system.

But either conclusion serves the purpose of the discussion equally well. For what Mr. Spencer wished to show was, that the relations of the comets to the solar system are not utterly fortuitous and irregular, but such as indicate a systematic connection; and this is undoubtedly true, since the connection of the planetary and cometary orbits is even more direct and intimate than Mr. Spencer has suspected. The inference which Arago’s table warrants is, then, another in that interesting series of facts which some physical theory, whether nebular or not, by “evolution” or by involution, may some day explain.

The greater number of the arguments, old and new, which Mr. Spencer adduces in support of his thesis, do not apply specifically to the nebular hypothesis in particular, but are simply an enumeration of the facts which go to show the existence of physical connections, of an unknown origin and species, in the solar system. In his handling of the mechanical problems of the nebular genesis, Mr. Spencer has succeeded no better than his predecessors. In attempting to account for the exceptions to a general law which the rotations of the outer planets, Uranus and Neptune, and the revolutions of their satellites, exhibit,—the great inclinations of the planes of these rotations and revolutions to the planes of the orbits of the primaries,—Mr. Spencer makes what appears to us a very erroneous assumption, and one from which the conclusion he wishes to draw by no means inevitably follows.

It is one of the few successes of the nebular hypothesis, that it accounts in a general way for the fact that the planes and directions of the rotations of the planets, and the revolutions

of their satellites, nearly coincide with the planes and directions of their own orbital motions. A ring of nebulous matter, detached by its centrifugal force from the revolving mass of the nebula, contains within it the conditions by which the direction, and even the amount, of the rotation of the resulting planet is determined; and this direction is the same as that of the revolution of the ring. The ring must originally be of a very thin, quoit-shaped form, even if it be composed of separate, independently moving parts; otherwise the planes of the orbits of the several parts would not pass through or near to the centre of attraction in the central nebula, and the parts must either pass through each other from one to the other surface of the ring, which would tend, along with other forces, to flatten it to the requisite thinness. Hence, a hoop- -shaped fluid ring, or one thinner in the directions of its radii than in a direction perpendicular to its general plane, could not exist. Much less could such a ring be detached by its self-sustaining centrifugal force from the body of the nebula. The nebula must necessarily be flattened in its equatorial regions to a sharp, thin edge by the centrifugal force of its revolution, before those regions could be separated to form a ring. The supposition, therefore, which Mr. Spencer’s ingenuity has devised to account for the anomalies presented in the rotations and the secondary systems of Uranus and Neptune,—a hoop-shaped ring, with a less determinate tendency to rotation in forming a planet,—is untenable. But this is not all. Supposing such a form possible, and even if the parts of the ring did not move among themselves, or press upon one another so as to flatten the ring, yet the direction of its tendency to rotation in contracting to a planet is just as determinate as in the quoit-shaped ring.

We have gone thus into detail, to show the vague and uncertain character of the mechanical arguments of the nebular hypothesis when they deal with details in the constitution of the solar system. In his treatment of recent discoveries and views in stellar astronomy, we think Mr. Spencer more fortunate. We agree with him in believing the current opinion to be an error, which represents the nebulæ as isolated

sidereal systems, inconceivably remote, and with magnitudes commensurate with the Galactic system itself. There are many reasons for believing that the nebulæ belong to this system, and that they are, in general, at no greater distance from us than the stars themselves. We think, also, with him, that the actual magnitudes of the stars are probably of all degrees, and that their apparent magnitudes do not generally indicate their relative distances from us. We would even go further, and maintain, as both a priori most probable, and most in accordance with observation, that the free bodies of the universe range in size from a grain of dust to masses many times larger than the sun, and that the number of bodies of any magnitude is likely to bear some simple proportion to the smallness of this magnitude itself. Star-dust is not at all distasteful to us, except in the form of nebular boluses. For reasons which will appear hereafter, the smaller bodies are not likely to be self-luminous; and star-dust is probably the cause of more obscuration than light in the stellar universe. That gaseous and liquid masses also exist with all degrees of rarefaction or density, dependent on the actions of heat and gravitation, is also, we think, very probable; and the three states of aggregation in matter doubtless play important parts in the cosmical economy.

Before leaving Mr. Spencer, to attend more immediately to the merits of the nebular hypothesis, we wish to adopt from him an estimate of the value of certain ideas in geology, the bearing of which on our subject is not so remote as it may at first sight appear to be.

Geology has not yet so far detached itself from cosmological speculations as to be entitled to the rank of a strictly positive science. The influence of such speculations upon its terminology, and upon the forms of the questions and the directions of the researches of its cultivators, is still very noticeable, and shows how difficult it is to start anew in the prosecution of physical inquiries, or completely to discard unfounded opinions which have for a long time prevailed. Greater sagacity is sometimes required to frame wise questions, than to find their answers. Geologists still continue to collate remote stratifications

as to their stratigraphical order, mineral composition, and fossil remains, as if these were still expected to disclose a comparatively simple history—simple at least in its outlines —of the changes which the life of our globe has undergone. A story, dramatically complete from prologue to epilogue, was demanded in the cosmological childhood of the science, and its manhood still searches in the fragmentary and mutilated records for the history of the creation. But doubtless the story is as deficient in the dramatic unities, as the record itself is in continuity or completeness. Referring to Mr. Spencer’s admirable essay on “Illogical Geology” for our reasons, we will simply state our belief that nothing in the form of a complete or connected history will ever be deciphered from the geological record.

“Only the last chapter of the earth’s history has come down to us. The many previous chapters, stretching back to a time immeasurably remote, have been burnt, and with them all the records of life we may presume they contained. The greater part of the evidence which might have served to settle the development controversy is forever lost; and on neither side can the arguments derived from geology be conclusive.”

We must not ascribe to Mr. Spencer, however, our opinion, that, even if this record were more complete, we should not necessarily be the wiser for it. According to Mr. Spencer’s views, the first strata, had they been preserved, would have contained the remains of protozoa and protophytes; but, for aught we dare guess, they might have contained the footprints of archangels.

Evidence of progress in life through any ever so considerable portion of the earth’s stratified materials would not, in our opinion, warrant us in drawing universal cosmical conclusions therefrom. Alternations of progress and regress relatively to any standard of ends or excellence which we might apply, is to us the most probable hypothesis that the general analogy of natural operations warrants. Nevertheless, as we have already intimated, we accept the purely physical portion of the “development hypothesis,” both in its astronomical and biological applications, but would much prefer to designate the doctrine in both its applications by the name we have already quoted. This name, “the derivative hypothesis,” simply connotes

the fact that, in several classes of phenomena hitherto regarded as ultimate and inexplicable, physical explanations are probable and legitimate. But it makes no claim to rank with the names of the Muses as a revealer of the cosmical order and the beginning of things.

We are aware that in thus summarily rejecting the cosmological import of the nebular hypothesis, along with its special physical assumptions, and retaining only its fundamental assumption, that the solar system is a natural product, we leave no provision to meet a demand which we allow, and we ought to justify this insolvency by proving the bankruptcy of the hypothesis whose debts we thus assume. It would be difficult, however, to prove that this hypothesis cannot fulfill the promise it has so long held out. Much more difficult would it be to supply its place with an equally plausible theory. But our object should not be to satisfy the imagination with plausibility. If we succeed in satisfying our understanding with the outlines of a theory sufficiently probable, we shall have done all that in the present state of our knowledge can reasonably be demanded.

The agencies of heat and gravitation acting, however slowly, through the ages of limitless time, and according to the law of counter-movements, or according to the analogy of the weather, constitute the means and the general mode of operation from which we anticipate an explanation of the general constitutions of solar and sidereal systems.

There comes to our aid a remarkable series of speculations and experiments recently promulgated upon the general subject of the nature and origin of heat, and under the general name of “The Dynamical Theory of Heat,” the principles of which we shall endeavor briefly to explain. It is a fundamental theorem in mechanical philosophy, that no motion can be destroyed, except by the production of other equivalent motions, or by an equivalent change in the antecedent conditions of motion. If we launch a projectile upward, the motion which we impart to it is not a new creation, but is derived from forces or antecedent conditions of motion of a very complicated character in our muscular organism. It would be

confusing to consider these at the outset; but if we look simply to the motion thus produced in the projectile itself, we shall gain the best preliminary notions as to the character of the phenomena of motion in general. The projectile rises to a certain height and comes to rest, and then, unless caught upon some elevated support, like the roof of a house, it returns to the ground with constantly accelerated motion, till it is suddenly brought to rest by collision with the earth. In this series of phenomena we have in reality only a series of commutations of motions and conditions of motion. The projectile is brought to rest at its greatest elevation by two forms of commutation. A small part of its motion is given to the air, and the remainder is transformed into the new condition of motion represented by its elevated position. The latter may remain for a long time permanent in case the projectile is caught at its greatest elevation upon some support. But a small auxiliary movement dislodging the projectile may at any time develop this condition of motion into a movement nearly equal to that which the projectile first received from our muscles. The small part that is lost in the air or other obstacles still exists, either in some form of motion or in some new conditions of motion, and the much greater part which disappears in the collision of the projectile with the earth is converted into several kinds of vibratory molecular movements in the earth, in the air, and in the projectile itself; and perhaps in part also in various new molecular conditions of motion.

If we designate by the word “power” that in which all forms of motion or antecedent conditions of motion are equivalent, we find that in the operations of nature no “power” is ever lost. Nor is there any evidence that any new “power” is ever created. It would be foreign to our purpose to follow into their ramifications the speculations by which this interesting theorem has been illustrated in many branches of physical inquiry. We are immediately interested only in the three principal and most general manifestations of “power” in the universe, namely, the movements of bodies, the movements in bodies, and the general antecedent conditions of both.

The proposition that the principal molecular motions in bodies are the cause which produces in our nerves the sensations of heat, or that they are what we denominate “the substance of heat,”—the objective cause of these sensations,— has long been held as a very probable hypothesis; and has latterly received experimental confirmations amounting to complete proof. The three principal manifestations of “power” in the universe are then, more specifically, the massive motions of bodies in translation and rotation, their molecular motions, or heat; and the principal antecedent condition of both, or gravitation.

In comparing these as to their equivalence we obtain a sum of “power,” which remains invariable and indestructible by the operations of nature. It remains to determine the precise relations of their equivalence, and what the operations are by which they are converted into each other.

The mechanical equivalent of heat is a quantity which has been very accurately determined by experiment. By means of it we may very readily compute what amount of heat would be produced if a given amount of massive motion were converted into heat by friction or otherwise; or conversely, what amount of massive motion could be produced by the conversion of a given amount of heat into mechanical effect; but it is unnecessary to our purpose to give the precise method of this computation.

The mechanical equivalent of gravitation is another quantity or relation depending on the changes of what is called the “potential” of gravitation, or the sum of the ratios of the masses to the distances apart of the gravitating bodies. The “power” of motion is a relation or quantity, commonly called the “living force” of motion, and depends on the mass and on the square of the velocity of the moving body.

The living forces of all moving bodies, minus the potentials of their forces of gravitation, plus the mechanical values of their heat, equal to a constant quantity,—is the precise formula to which our cosmical speculations should conform. It will be impossible, however, to make any other than a very general use of this precise law. What concerns us more

nearly is the consideration of the natural operations by which these manifestations of “power” are converted into each other.

The origin of the sun’s light and heat is a problem upon which speculative ingenuity has long been expended in vain. The metaphysical conclusion, that the sun is composed of pure fire, or of fire per se, the very essence of fire, is one of many illustrations of the ingenious way in which speculation covers its nakedness with words, and can really mean, we imagine, only that the sun is very hot. That the sun, like any other body, must grow cooler by the expenditure of heat, is without doubt an indisputable proposition; and the question, how this heat is restored to it, is thus a legitimate one. The nebular hypothesis explains how the primitive heat in the sun and in other bodies could be generated by the condensation of the original nebulous mass, in which the heat is supposed to have been originally diffused; but it affords no explanation of the manner in which this heat could be sustained through the ages that must have elapsed since the nebular genesis must have been completed.

There are no precise means of estimating the amount of heat contained in the sun, since the capacity for heat of the materials which compose it are unknown; but from general analogy it may safely be assumed that the sun must grow cooler at a sensible rate, unless its heat is in some way renewed. Concerning the rate of its expenditure of heat, and the means which the dynamical theory of heat proposes to supply the loss, we will quote from the interesting lectures of Professor Tyndall, “On Heat considered as a Mode of Motion.”

“The researches of Sir J. Herschel and M. Pouillet have informed us of the annual expenditure of the sun as regards heat, and by an easy calculation we ascertain the precise amount of the expenditure which falls to the share of our planet. Out of 2,300 million parts of light and heat the earth receives one. The whole heat emitted by the sun in a minute would be competent to boil 12,000 millions of cubic miles of icecold water. How is this enormous loss made good? Whence is the sun’s heat derived, and by what means is it maintained? No combustion, no chemical affinity with which we are acquainted, would be competent to produce the temperature of the sun’s surface. Besides, were the sun

a burning body merely, its light and heat would assuredly speedily come to an end. Supposing it to be a solid globe of coal, its combustion would only cover 4,600 years of expenditure. In this short time it would burn itself out. What agency, then, can produce the temperature and maintain the outlay? We have already regarded the case of a body falling from a great distance towards the earth, and found that the heat generated by its collision would be twice that produced by the combustion of an equal weight of coal. How much greater must be the heat developed by a body falling towards the sun! The maximum velocity with which a body can strike the earth [arising from the earth’s attraction] is about 7 miles a second; the maximum velocity with which it can strike the sun is 390 miles a second. And as the heat developed by the collision is proportional to the square of the velocity destroyed, an asteroid falling into the sun with the above velocity would generate about 10,000 times the quantity of heat generated by the combustion of an asteroid of coal of the same weight.

“Have we any reason to believe that such bodies exist in space, and that they may be rained down upon the sun? The meteorites flashing through our air are small planetary bodies, drawn by the earth’s attraction, and entering our atmosphere with planetary velocity. By friction against the air they are raised to incandescence, and caused to emit light and heat. At certain seasons of the year they shower down upon us in great numbers. In Boston [England] 240,000 of them were observed in nine hours. There is no reason to suppose that the planetary system is limited to vast masses of enormous weight; there is every reason to believe that space is stocked with smaller masses, which obey the same laws as the large ones. That lenticular envelope which surrounds the sun, and which is known to astronomers as the zodiacal light, is probably a crowd of meteors; and, moving as they do in a resisting medium, they must continually approach the sun. Falling into it, they would be competent to produce the heat observed, and this would constitute a source from which the annual loss of heat would be made good. The sun, according to this hypothesis, would be continually growing larger; but how much larger? Were our moon to fall into the sun, it would develop an amount of heat sufficient to cover one or two years’ loss; and were our earth to fall into the sun, a century’s loss would be made good. Still, our moon and our earth, if distributed over the surface of the sun, would utterly vanish from perception. Indeed, the quantity of matter competent to produce the necessary effect would, during the range of history, produce no appreciable augmentation of the sun’s magnitude. The augmentation of the sun’s attractive force would be more appreciable. However this hypothesis may fare as a representant of what is going on in nature, it certainly shows how a sun might be formed and maintained by the application of known thermo-dynamic principles.” Appendix to Lecture XII. p. 455. ⁷

This part of our inquiry—how gravitation and motion are converted into heat—is receiving the amplest illustration and discussion from physicists at the present time; and, though the somewhat startling conclusions we have quoted are still too new to be generally credited, they are too well founded in experiment and the general analogies of natural phenomena to be passed lightly by.

The second part of our inquiry—how heat is refunded, in the eternal round of cosmical phenomena, into the antecedent conditions of motion, or to the conditions which preceded the production of the motions that are converted into heat—is a subject to which physicists have given little attention. Indeed, the cosmological ideas which prevail in geological inquiries beset this subject also, and impede inquiry. The order of nature is almost universally regarded as a progression from a determinate beginning to a determinate conclusion. The dynamical theory of heat lengthens out the process better, perhaps, than the nebular hypothesis alone; but both leave the universe at length in a hopeless chaos of huge, dark masses,—ruined suns wandering in eternal night.

It seems not to have occurred to physicists to inquire what becomes of the heat the generation of which requires so great an expenditure of motion. The heat is, in another form, the same motion as that which is lost by the fallen bodies. It is radiated into space, while the bodies remain in the sun; but this radiation is still the same motion in other bodies, in the luminiferous ether, or in the diffused matters of space. It cannot be lost from the universe, and must either accumulate in diffused materials or be converted into other motions or into new conditions of motion. But if the solid bodies of the universe are gradually collected at certain centres, and their motions are diffused in the form of heat throughout the gaseous materials of space, what do we gain? How do we by such a conclusion avoid the ultimate catastrophe which we regard as the reductio ad absurdum of a scientific theory? How do we thereby constitute that cycle of movements which we regard as characteristic of all natural phenomena? Perhaps we have been somewhat too hasty in adopting the conclusion that the

fallen bodies must necessarily remain in the sun, and grad ually augment its mass. Let us, therefore, examine this point more closely.

The principles of the steam-engine afford a clew to the converse process we are in search of, by which heat may be refunded into mechanical effects and conditions. The mechanical effects of the expanding power of steam are only partially developed in the work which the engine performs. This work, converted back to heat by friction or otherwise, would be insufficient to reproduce the same effects in the form of steam. The remaining power consists in the motions and the power of expansion with which the steam escapes from the engine. This is lost power; but if it should be allowed to develop itself by an expansion of the steam into an indefinitely extended vacuum, the molecular motions of the particles of the steam would gradually, and on the outside of the expanding vaporous mass, be converted into velocities or massive motions; the vapor itself would be converted back into water, or even be frozen into snow, and the particles of this water or snow would, at the top of the expanding cloud, finally come to rest by the force of gravitation. A part, therefore, of the lost power of the heat which escaped in steam would be converted into that antecedent condition of motion represented by elevation above the attracting mass of the earth or by gravitation; a part would continue to manifest itself as velocity or massive motion; and the remainder would still continue to exert an outward pressure in the form of heat in vapor. This development would continue so long as the steam continued to discharge itself into the indefinitely extended vacuum we have supposed. The rain or snow falling from the top of the cloud would convert its gravitative power back again into motion, which, again arrested by collision with the earth, would suffer other transformations in the endless round. In the actual case, where the steam escapes into the air instead of a vacuum, the phenomena would be less simple. The history of its heat would become involved with the grander phenomena of the weather, —phenomena that may be regarded as typical of that cosmical weather, concerning the laws of which we must inquire in considering what becomes of the sun’s heat.

This heat is capable, provided it could all be so expended, of lifting the amount of matter which, by falling into the sun, is supposed to produce it, to the same height from the sun as that from which the fallen bodies may be supposed to have descended. This follows from the general mechanical principles we have stated. But how is this lifting effected? What is the Titanic machinery by which the sun performs this labor? The velocity with which a body falling from the interstellar spaces enters the body of the sun is sufficient, when converted to heat by friction and the shock, to convert the body itself into vapor, even if the body be composed of the least fusible of materials. The heat thus produced is not, however, confined to the fallen matter. A large portion is imparted to the matter already in the sun; but parts, no doubt, both of the projectile and of the resisting material are vaporized. The atmosphere immediately surrounding the sun contains the vapors of many of the most refractory metals that are known, as we learn from that wonderful instrument, the spectroscope. And this is made evident by the absorption from the sun’s luminous rays of certain portions characteristic of these metals. Doubtless, in absorbing their characteristic vibrations, these metals are further heated and expanded, and gradually lifted from the surface of the sun; and the vibrations of light and heat that pass through them and escape are probably all ultimately absorbed in the same or some similar way in the diffused materials of space. The speculations of the elder Struve on the extinction of light in its passage through space—conclusions founded on Sir William Herschel’s observations of the Milky-Way—afford a happy and independent confirmation of these views. Moreover, the spectroscopic analyses of the light of the stars show broad dark bands, indicative of great extinctions of light. And we may add, that many gases and vapors which are transparent to luminous rays are found to absorb the obscure rays of heat.

Such is the kind of evidence we have of what becomes of the light and heat, and a portion, at least, of the material of the sun. The heat which is not expended immediately in vaporizing these materials is ultimately extinguished in further heating,

expanding, and thus lifting the materials (may we not believe? ) which have already been partially raised to the height, whence perhaps, in former ages, they in their turn were rained down as meteors upon the sun. In these suppositions we have exactly reversed the nebular hypothesis. Instead of, in former ages, a huge gaseous globe contracted by cooling and by gravitation, and consolidated at its centre, we have supposed one now existing, and filling that portion of the interstellar spaces over which the sun’s attraction predominates,—a highly rarefied continuous gaseous mass, constantly evaporated and expanded from its solid centre, but constantly condensed and consolidated near its outer limits,—constantly heated at its centre by the fall of solid bodies from its outer limits, and constantly cooled and condensed at these limits by the conversion of heat into motion and the arrest of this motion by gravitation.

There are certain chemical objections which apply equally to the views here advanced and to the nebular hypothesis. But these must necessarily arise from the limits to the knowledge we can gain of the whole range of chemical phenomena. For what takes place in the chemist’s laboratory, under the very limited conditions of temperature and pressure he can command, ought not to be regarded as determining the possibilities, or even the probabilities, of that cosmical chemistry of which we can hardly be supposed to know even the rudiments. We shall consider this subject, however, more particularly, after attending to what is now of more immediate interest, namely, the secondary mechanical conditions and phenomena that result from the suppositions we have made; and particularly the question, how the systems of the planets and their satellites stand related to the round of changes we have considered.

The fundamental and most important motions of the solar system are, as we suppose, the radial movements of solid bodies inward and of gaseous bodies outward, arising from the counteractions of gravitation and heat. But these radial movements must assume a vortical form, if one does not already exist, such as is constantly exhibited by movements in the air and in water. The rotation of the sun, imparted to the materials

which rise in vapor from its surface, continues in them as they rise higher and higher, and though exhibited in a constantly diminishing tangential motion, remains in reality constant, as measured by what mechanicians term “rotation area.” Or, rather, it is slowly increased by the mutual resistances of contiguous strata in the expanding gases, so that when this matter falls again towards the sun in the form of solid bodies, it falls in spiral trajectories, and only reaches the sun after perhaps many revolutions, or not at all, unless its motions be rapidly diminished by the resisting medium. If the resistance of the medium is not sufficient to convert the path of a falling meteor into a spiral, the meteor will mount again, and continue to move perhaps for a long time in an eccentric orbit, like a comet. When, however, the meteor at length, in any way, reaches the sun, a part of its motion is expended in increasing the sun’s rotation, and thus compensating the loss of motion continually sustained by the sun in the evaporation of its material. The denser the resisting medium is in any system, the greater will be the revolution of its outer parts, and the larger will be the spiral trajectories which its falling bodies will describe. Such spiral or vortical motions as would thus be produced, or rather sustained, in the matter surrounding the sun, is exhibited by the most powerful telescopes, in the forms of the appendages to certain nebulous stars, and in the structure of the so-called Spiral Nebulæ. Perhaps the bodies which are supposed to give rise to the appearance of the zodiacal light would exhibit some such spiral arrangement, if seen from a point far above or below the ecliptic.

It follows from this vortical motion, that the form which the diffused materials of the solar system would assume, or rather maintain, would be that of an oblate ellipsoid or of a flattened lenticular body. The height to which the matter would rise in the plane of the sun’s equator before its massive and molecular motions would be arrested by gravitation, would be much greater than in the directions of the sun’s axis of rotation. The degree of oblateness which such a system of diffused matter will maintain depends on the frictions or resistances that successive strata exert on each other. It should be borne in

mind in this connection that friction is not a loss of force, where all kinds of force are taken into account. Friction or resistance can only effect a conversion of massive into molecular motions, or the motion of velocity into the motion of heat. Hence, whatever velocity is lost by interior strata in the gaseous materials of the solar system, and is not gained by those exterior to them, must yet be ultimately restored; for the stability of such a system is no longer a question; this is insured in the fundamental mechanical law on which our speculations are founded.

It may still be a question, however, whether the planetary bodies of such a system are successively produced and destroyed, like generations of animals and plants, or whether they are permanent elements in a system of balanced forces and operations. So far as the effects of mutual perturbation are concerned, and independently of a resisting medium, astronomers have shown that the latter supposition is the more probable one; but there are several other considerations which point to a different conclusion. In the first place, the considerations already mentioned. The existence of systematic relations in the structure of the solar system, some of which are independent of its stability under the law of gravity, indicate the operations of causes other than the simple ones on which this stability depends,—such causes as the nebular hypothesis endeavored to define, but which we, in rejecting this hypothesis, have still to search for.

It has undoubtedly occurred to our readers to ask how the planets stand related to the meteoric system, and in what manner, if at all, their motions and masses are affected by this perpetual shower of matter. As out of every two thousand million parts of the light and heat of the sun’s radiation the earth receives one part, so out of the two thousand million meteors sent back in return the earth will receive one, or perhaps a somewhat larger proportion, since the meteors are supposed to fall most thickly near the plane of the sun’s equator. If we multiply this proportion by ten, as we probably may, it is still a very small quantity; but if we are permitted to multiply it by a factor of time as great as we please, this insignificance

will disappear, and in its place we shall have a cosmical cause of the greatest moment in the history of the solar system. Two hundred million years is but a day in the cosmical eras, yet in that time the earth could receive as many bodies as fall to the sun in a year, or a hundredth part of the mass of the earth itself. In a hundred such days, then, the earth might be built up by the aggregation of meteors, provided it should lose none of the material thus collected, as the sun probably does. But this calculation proceeds on the supposition that the earth would have caught as many meteors when it was smaller, as it probably does now. A correction is therefore required which lengthens the period to three hundred such days, or to about a cosmical year, if we may so estimate times which are without limits or measure. In sixty thousand million years, then, the earth could have been made by the aggregation of meteors. Most of the materials which fall to the earth are probably in the form of very small bodies, which must be disintegrated by heat in their passage through the atmosphere, and must consequently reach the earth’s surface in the form of fine dust. At the rate of accumulation estimated above, this dust, when reduced to the mean density of the earth’s materials, would add one foot to the thickness of its crust in about three thousand years. In the loose form of dust or mud this accumulation would amount to about a hundredth of an inch in a year. The materials which have accumulated within historical periods over the ruins of ancient cities may thus in great part have been collected from the sky. The agencies of the winds and of flowing water in transporting and depositing the loose materials of the earth’s surface would distribute this star-dust in deposits at the bottom of the sea, and in hills and mounds on the land. ⁸ In this time the sun itself would have received and evaporated fifteen hundred times the amount of its present mass, provided a permanent amount of matter and heat should have been maintained in it during so long a period. In these estimates no account is taken of the heat immediately absorbed in evaporation, or absorbed in the space included within the earth’s orbit. This heat would probably require a still greater expenditure of motion, and the fall of a still greater number of bodies. Hence the period required to build up the earth’s mass might be materially shortened.

Such a method of inquiry, however, violates the canon we have laid down for our guidance in physical speculation. We must not suppose any action in nature to which there is not some counteraction, and no mode of production, however slow, from which in infinite time there could result an infinite

product. We must, therefore, conclude that the planets either ultimately fall into the sun, and make a restitution of their peculations, or that heat and gravitation preserve in them also the balance of nature and the golden mean of virtue. The existence of a resisting medium favors the first supposition, unless it can be rendered probable that this medium revolves with velocities equal to those of the planets at the same distances from the sun. There is also another cause affecting the mean distances of the planets. An increase of mass in the sun will diminish the size of the planetary orbits, and conversely a diminution of this mass will increase the size of these orbits. The rate of change in the mass of the sun, whether to increase or to decrease, must depend on the relative rates of cooling by radiation and by evaporation. As the sun grows cooler by excessive radiation, its mass must be increased by the fall of meteors, and the planets will draw nearer to the sun; but if its radiation be diminished, and a larger proportion of the heat be expended in evaporation, then the planets will withdraw from the sun. Such are the causes which may affect the mean distances of the planets.

If on such grounds we may adopt the first of our suppositions, that the planets are successively formed and finally lost in the sun, like the meteors, the most probable hypothesis we can make concerning their origin is, that they are formed by the aggregation of meteors. Certain conditions, which, in the present state of our knowledge, it would perhaps be impossible to define, must determine the distances from the sun where these aggregations will begin; but the body and the attraction of the planet, when once begun, will determine further aggregation until the planet either falls into the sun, or approaches to such a distance that the evaporation of its material keeps pace with the fall of matter upon it. The size to which a planet could attain would thus be determined by the distance from the sun at which it begins to grow. A nearly circular orbit, and a small inclination of its plane to the plane of the sun’s equator, would result from the circumstances attending the fall of the meteors,—their approach to the sun from every

direction near the plane of the sun’s equator. The rare occurrence of spots on the sun beyond thirty degrees either side of its equator may indicate some connection between these spots and the fall of meteors and serve to determine the limits of the meteoric system. ⁹ A vortical motion and a rotation of the planet might result from such aggregations, which would be analogous to those of the sun and the general system. A more rigorous and comprehensive discussion of such problems than has yet been attempted is required before trustworthy conclusions can be formed.

The following considerations may materially affect the conclusions we have drawn from the existence of a resisting medium. The gaseous medium of the solar system might receive from the sun’s rotation, and by the mutual friction of its own materials, greater velocities in its interior parts than the planets could have at the same distances from the sun, provided the exterior parts should move with less than planetary velocities, and should press with a portion of their weight upon the parts below them. For the centrifugal forces of the interior parts might thus be balanced, not merely by their own gravitation, but by a portion also of the weight of the superincumbent masses. At a distance from the sun less than half the mean distance of the planet Mercury, a period of revolution equal to that of the sun would produce a planetary velocity. At a greater distance, the medium might revolve more rapidly than the planets. But there must be a limit where the revolutions would be simply self-sustaining, and beyond this the medium would move less rapidly than the planets. So far, therefore, as a resisting medium could affect the motions of the planetary bodies, it might tend to increase the dimensions of the interior orbits, and to diminish those of the exterior ones; and it would thus tend to concentrate the planets, not in the sun, but at this limiting distance, where the medium would neither accelerate nor retard their motions. The motions of the medium would produce the greatest effect upon the smaller bodies of the solar system, which would, therefore, approach most rapidly to this limiting distance. That region in the solar system, about half the distance from the sun to the orbit of Jupiter, which is so thickly crowded with small planetary bodies or asteroids, may

be regarded, on this hypothesis, as the region in which the gaseous medium now revolves with planetary velocity. Could this limiting distance remain fixed for a very long period, most of the planetary masses of the solar system might accumulate there, and be concentrated into one huge planet or secondary sun, and the solar system would thus be converted into a binary system, like those observed among the stars. But from the small amount of matter probably contained in the asteroid system, we ought to conclude that this limiting distance changes from time to time, as the medium grows denser or rarer.

The planets are not the only aggregations of meteoric bodies which we have to account for. Besides the comets, there are probably streams of meteors falling to or circulating around the sun. This is rendered very probable by the phenomena of the showers of these bodies which fall into our atmosphere at certain seasons of the year, or at certain positions in the earth’s orbit. There is a period of about eleven years in the numbers of spots that appear on the surface of the sun, a period coincident with that of the amount of diurnal variations in terrestrial magnetism,—an amount undoubtedly due to the influence of the sun. This period also coincides nearly with the period of the revolution of Jupiter, the largest planet in our system. If, then, we may suppose that the sun’s spots are occasioned by the fall of large meteors, the courses of which lie near to the orbit of Jupiter, the attractions of this planet, alternately turning such a stream of bodies upon and away from the surface of the sun, would connect these three nearly coincident periods by a common physical cause. The phenomena of magnetism and electricity, as subordinate manifestations of motion and conditions of motion, have not been included in our speculations on the commutations of “power,” on account of their insignificant values as compared with the three principal forms of “power.” For the same reason, we omit any consideration of the numerous but minute modifications of “power” which are manifested by the forces of vital phenomena on the surface of the earth. ¹⁰ And further, the rings of Saturn are probably examples of the same kind of meteoric aggregation. For of the three hypotheses in regard to the constitution of these rings which have been submitted to rigorous mathematical examination,— namely, first, that they are solid, secondly, that they are fluid, and, thirdly, that they are composed of distinct bodies or meteors,—the latter is the only one which has been found to afford the conditions of stability which are implied in their continued existence. It is unnecessary to add the physical reasons which render this hypothesis still more probable.

We have no space to consider the many interesting geological

consequences which follow from our hypothesis. Let it suffice to remark, that the formation of the earth’s mass by meteoric aggregation precludes the hypothesis, otherwise improbable that the core of the earth is a molten mass. The occurrence of volcanoes in local systems, distinct from each other, points to local causes of an unknown chemical character as the true sources of these phenomena. The heterogeneous character of the materials of the earth’s crust, in which are mingled, in the most intimate manner, all kinds of substances, irrespectively of their chemical affinities, and in opposition to their chemical forces of aggression, could hardly be the results of the actions of heat and aqueous solution, both of which afford conditions favorable to chemical aggregation. Indeed, in most cases in which such aggregation occurs, where homogeneous and chemically simple substances are found in considerable quantities, the agency either of heat or aqueous solution is evident. It is hardly necessary to add, that the theory of meteoric aggregation is the one which would most readily explain these facts.

But we must here leave the consideration of these interesting problems, and return to a topic much more obscure, to which we called attention a few pages back.

The dynamical theory of heat has not only suggested new and interesting inquiries concerning the constitution of the universe, but it throws new light in the philosophy of chemical phenomena on such problems as the origin of the three states of aggregation in matter, and on the character of the changes which may take place under circumstances beyond the reach of chemical experiments and observation.

That the dreams of the alchemists were at fault rather in point of method than of doctrine, is a confession which the modern chemist must make, when he compares the slight resources of experiment at his command with the possibilities of nature. If, as has been surmised, the characteristic properties of different kinds of matter consist in characteristic internal or molecular motions (and molecular conditions of motion), a complete destruction of such motions would obliterate all the characteristic differences of matter, and such a result might be attained by the production of absolute cold. In respect to

the motions of light and heat, however, the universe, so far as we know it, and even so far as we could know it, is a perfectly continuous body. In no comer or recess of its unfathomable depths to which the feeblest light of a single star could find its way, can there be an absence of the motions of light and heat. Nothing can set bounds to the all-pervading reach of these motions except limits to that medium of motion, the luminiferous ether; and these, so far as all cognizable physical conditions are concerned, would be limits to space itself. That potent sidereal influence, the absolute cold, transmuting all substances into one, could only arise momentarily, in nodal points or lines or surfaces, but could not be extended discontinuously into space of three dimensions. What may happen at such times and limits, where matter, expiring from one form of chemical life, may be awakened to another, according to the kind of molecular agitation which may next overtake it, and determine its history, perhaps for myriads of years, is what the chemist cannot tell us, and only the alchemist can dream. It suffices for our instruction, that the chemistry of absolute cold has possibilities of which experimental chemistry affords no criterion, and may play a part in the economy of nature not inferior to that of gravitation or heat.

But it may be objected, on grounds of experimental chemistry, “that the sun’s heat, though sufficient to volatize the least fusible materials, could not keep them in the form of vapor at the heights and in the temperature of the interplanetary spaces, much less lift them in the form of vapor to the heights of the interstellar regions whence the meteors are supposed to fall. For most bodies which are solid at ordinary terrestrial temperatures tend, upon cooling, to crystallize with such energy that they would soon be precipitated from the vaporous form.” But this objection takes no account of those effects of diffusion, expansion, and commingling of heterogeneous materials, which must remove the parts of a volatilized body to such hopeless distances from each other that the forces of chemical aggregation might require ages to collect what is thus dispersed. Nor can any account be taken of such unknown laws of chemical affinity and aggregation as are possible

under the circumstances we are considering. The known laws of chemical action should, then, be ranked with those laws of life, exhibited in the phenomena of growth, which were too hastily generalized and applied, in “the theory of evolution,” to the interpretation of the riddles and the explication of the order of the System of the World.