The Unseen World, and Other Essays

PART FIRST.

"What are you, where did you come from, and whither are you bound?"—the question which from Homer's days has been put to the wayfarer in strange lands—is likewise the all-absorbing question which man is ever asking of the universe of which he is himself so tiny yet so wondrous a part. From the earliest times the ultimate purpose of all scientific research has been to elicit fragmentary or partial responses to this question, and philosophy has ever busied itself in piecing together these several bits of information according to the best methods at its disposal, in order to make up something like a satisfactory answer. In old times the best methods which philosophy had at its disposal for this purpose were such as now seem very crude, and accordingly ancient philosophers bungled considerably in their task, though now and then they came surprisingly near what would to-day be called the truth. It was natural that their methods should be crude, for scientific inquiry had as yet supplied but scanty materials for them to work with, and it was only after a very long course of speculation and criticism that men could find out what ways of going to work are likely to prove successful and what are not. The earliest thinkers, indeed, were further hindered from accomplishing much by the imperfections of the language by the aid of which their thinking was done; for science and philosophy have had to make a serviceable terminology by dint of long and arduous trial and practice, and linguistic processes fit for expressing general or abstract notions accurately grew up only through numberless failures and at the expense of much inaccurate thinking and loose talking. As in most of nature's processes, there was a great waste of energy before a good result could be secured. Accordingly primitive men were very wide of the mark in their views of nature. To them the world was a sort of enchanted ground, peopled with sprites and goblins; the quaint notions with which we now amuse our children in fairy tales represent a style of thinking which once was current among grown men and women, and which is still current wherever men remain in a savage condition. The theories of the world wrought out by early priest-philosophers were in great part made up of such grotesque notions; and having become variously implicated with ethical opinions as to the nature and consequences of right and wrong behaviour, they acquired a kind of sanctity, so that any thinker who in the light of a wider experience ventured to alter or amend the primitive theory was likely to be vituperated as an irreligious man or atheist. This sort of inference has not yet been wholly abandoned, even in civilized communities. Even to-day books are written about "the conflict between religion and science," and other books are written with intent to reconcile the two presumed antagonists. But when we look beneath the surface of things, we see that in reality there has never been any conflict between religion and science, nor is any reconciliation called for where harmony has always existed. The real historical conflict, which has been thus curiously misnamed, has been the conflict between the more-crude opinions belonging to the science of an earlier age and the less-crude opinions belonging to the science of a later age. In the course of this contest the more-crude opinions have usually been defended in the name of religion, and the less-crude opinions have invariably won the victory; but religion itself, which is not concerned with opinion, but with the aspiration which leads us to strive after a purer and holier life, has seldom or never been attacked. On the contrary, the scientific men who have conducted the battle on behalf of the less-crude opinions have generally been influenced by this religious aspiration quite as strongly as the apologists of the more-crude opinions, and so far from religious feeling having been weakened by their perennial series of victories, it has apparently been growing deeper and stronger all the time. The religious sense is as yet too feebly developed in most of us; but certainly in no preceding age have men taken up the work of life with more earnestness or with more real faith in the unseen than at the present day, when so much of what was once deemed all-important knowledge has been consigned to the limbo of mythology.

The more-crude theories of early times are to be chiefly distinguished from the less-crude theories of to-day as being largely the products of random guesswork. Hypothesis, or guesswork, indeed, lies at the foundation of all scientific knowledge. The riddle of the universe, like less important riddles, is unravelled only by approximative trials, and the most brilliant discoverers have usually been the bravest guessers. Kepler's laws were the result of indefatigable guessing, and so, in a somewhat different sense, was the wave-theory of light. But the guesswork of scientific inquirers is very different now from what it was in older times. In the first place, we have slowly learned that a guess must be verified before it can be accepted as a sound theory; and, secondly, so many truths have been established beyond contravention, that the latitude for hypothesis is much less than it once was. Nine tenths of the guesses which might have occurred to a mediaeval philosopher would now be ruled out as inadmissible, because they would not harmonize with the knowledge which has been acquired since the Middle Ages. There is one direction especially in which this continuous limitation of guesswork by ever-accumulating experience has manifested itself. From first to last, all our speculative successes and failures have agreed in teaching us that the most general principles of action which prevail to-day, and in our own corner of the universe, have always prevailed throughout as much of the universe as is accessible to our research. They have taught us that for the deciphering of the past and the predicting of the future, no hypotheses are admissible which are not based upon the actual behaviour of things in the present. Once there was unlimited facility for guessing as to how the solar system might have come into existence; now the origin of the sun and planets is adequately explained when we have unfolded all that is implied in the processes which are still going on in the solar system. Formerly appeals were made to all manner of violent agencies to account for the changes which the earth's surface has undergone since our planet began its independent career; now it is seen that the same slow working of rain and tide, of wind and wave and frost, of secular contraction and of earthquake pulse, which is visible to-day, will account for the whole. It is not long since it was supposed that a species of animals or plants could be swept away only by some unusual catastrophe, while for the origination of new species something called an act of "special creation" was necessary; and as to the nature of such extraordinary events there was endless room for guesswork; but the discovery of natural selection was the discovery of a process, going on perpetually under our very eyes, which must inevitably of itself extinguish some species and bring new ones into being. In these and countless other ways we have learned that all the rich variety of nature is pervaded by unity of action, such as we might expect to find if nature is the manifestation of an infinite God who is without variableness or shadow of turning, but quite incompatible with the fitful behaviour of the anthropomorphic deities of the old mythologies. By thus abstaining from all appeal to agencies that are extra-cosmic, or not involved in the orderly system of events that we see occurring around us, we have at last succeeded in eliminating from philosophic speculation the character of random guesswork which at first of necessity belonged to it. Modern scientific hypothesis is so far from being a haphazard mental proceeding that it is perhaps hardly fair to classify it with guesses. It is lifted out of the plane of guesswork, in so far as it has acquired the character of inevitable inference from that which now is to that which has been or will be. Instead of the innumerable particular assumptions which were once admitted into cosmic philosophy, we are now reduced to the one universal assumption which has been variously described as the "principle of continuity," the "uniformity of nature," the "persistence of force," or the "law of causation," and which has been variously explained as a necessary datum for scientific thinking or as a net result of all induction. I am not unwilling, however, to adopt the language of a book which has furnished the occasion for the present discussion, and to say that this grand assumption is a supreme act of faith, the definite expression of a trust that the infinite Sustainer of the universe "will not put us to permanent intellectual confusion." For in this mode of statement the harmony between the scientific and the religious points of view is well brought out. It is as affording the only outlet from permanent intellectual confusion that inquirers have been driven to appeal to the principle of continuity; and it is by unswerving reliance upon this principle that we have obtained such insight into the past, present, and future of the world as we now possess.

The work just mentioned 1 is especially interesting as an attempt to bring the probable destiny of the human soul into connection with the modern theories which explain the past and future career of the physical universe in accordance with the principle of continuity. Its authorship is as yet unknown, but it is believed to be the joint production of two of the most eminent physicists in Great Britain, and certainly the accurate knowledge and the ingenuity and subtlety of thought displayed in it are such as to lend great probability to this conjecture. Some account of the argument it contains may well precede the suggestions presently to be set forth concerning the Unseen World; and we shall find it most convenient to begin, like our authors, with a brief statement of what the principle of continuity teaches as to the proximate beginning and end of the visible universe. I shall in the main set down only results, having elsewhere 2 given a simple exposition of the arguments upon which these results are founded.

The first great cosmological speculation which has been raised quite above the plane of guesswork by making no other assumption than that of the uniformity of nature, is the well-known Nebular Hypothesis. Every astronomer knows that the earth, like all other cosmical bodies which are flattened at the poles, was formerly a mass of fluid, and consequently filled a much larger space than at present. It is further agreed, on all hands, that the sun is a contracting body, since there is no other possible way of accounting for the enormous quantity of heat which he generates. The so-called primeval nebula follows as a necessary inference from these facts. There was once a time when the earth was distended on all sides away out to the moon and beyond it, so that the matter now contained in the moon was then a part of our equatorial zone. And at a still remoter date in the past, the mass of the sun was diffused in every direction beyond the orbit of Neptune, and no planet had an individual existence, for all were indistinguishable parts of the solar mass. When the great mass of the sun, increased by the relatively small mass of all the planets put together, was spread out in this way, it was a rare vapour or gas. At the period where the question is taken up in Laplace's treatment of the nebular theory, the shape of this mass is regarded as spheroidal; but at an earlier period its shape may well have been as irregular as that of any of the nebulae which we now see in distant parts of the heavens, for, whatever its primitive shape, the equalization of its rotation would in time make it spheroidal. That the QUANTITY of rotation was the same then as now is unquestionable; for no system of particles, great or small, can acquire or lose rotation by any action going on within itself, any more than a man could pick himself up by his waistband and lift himself over a stone wale So that the primitive rotating spheroidal solar nebula is not a matter of assumption, but is just what must once have existed, provided there has been no breach of continuity in nature's operations. Now proceeding to reason back from the past to the present, it has been shown that the abandonment of successive equatorial belts by the contracting solar mass must have ensued in accordance with known mechanical laws; and in similar wise, under ordinary circumstances each belt must have parted into fragments, and the fragments chasing each other around the same orbit, must have at last coalesced into a spheroidal planet. Not only this, but it has also been shown that as the result of such a process the relative sizes of the planets would be likely to take the order which they now follow; that the ring immediately succeeding that of Jupiter would be likely to abort and produce a great number of tiny planets instead of one good-sized one; that the outer planets would be likely to have many moons, and that Saturn, besides having the greatest number of moons, would be likely to retain some of his inner rings unbroken; that the earth would be likely to have a long day and Jupiter a short one; that the extreme outer planets would be not unlikely to rotate in a retrograde direction; and so on, through a long list of interesting and striking details. Not only, therefore, are we driven to the inference that our solar system was once a vaporous nebula, but we find that the mere contraction of such a nebula, under the influence of the enormous mutual gravitation of its particles, carries with it the explanation of both the more general and the more particular features of the present system. So that we may fairly regard this stupendous process as veritable matter of history, while we proceed to study it under some further aspects and to consider what consequences are likely to follow.

Our attention should first be directed to the enormous waste of energy which has accompanied this contraction of the solar nebula. The first result of such a contraction is the generation of a great quantity of heat, and when the heat thus generated has been lost by radiation into surrounding space it becomes possible for the contraction to continue. Thus, as concentration goes on, heat is incessantly generated and incessantly dissipated. How long this process is to endure depends chiefly on the size of the contracting mass, as small bodies radiate heat much faster than large ones. The moon seems to be already thoroughly refrigerated, while Jupiter and Saturn are very much hotter than the earth, as is shown by the tremendous atmospheric phenomena which occur on their surfaces. The sun, again, generates heat so rapidly, owing to his great energy of contraction, and loses it so slowly, owing to his great size, that his surface is always kept in a state of incandescence. His surface-temperature is estimated at some three million degrees of Fahrenheit, and a diminution of his diameter far too small to be detected by the finest existing instruments would suffice to maintain the present supply of heat for more than fifty centuries. These facts point to a very long future during which the sun will continue to warm the earth and its companion planets, but at the same time they carry on their face the story of inevitable ultimate doom. If things continue to go on as they have all along gone on, the sun must by and by grow black and cold, and all life whatever throughout the solar system must come to an end. Long before this consummation, however, life will probably have become extinct through the refrigeration of each of the planets into a state like the present state of the moon, in which the atmosphere and oceans have disappeared from the surface. No doubt the sun will continue to give out heat a long time after heat has ceased to be needed for the support of living organisms. For the final refrigeration of the sun will long be postponed by the fate of the planets themselves. The separation of the planets from their parent solar mass seems to be after all but a temporary separation. So nicely balanced are they now in their orbits that they may well seem capable of rolling on in their present courses forever. But this is not the case. Two sets of circumstances are all the while striving, the one to drive the planets farther away from the sun, the other to draw them all into it. On the one hand, every body in our system which contains fluid matter has tides raised upon its surface by the attraction of neighbouring bodies. All the planets raise tides upon the surface of the sun and the periodicity of sun-spots (or solar cyclones) depends upon this fact. These tidal waves act as a drag or brake upon the rotation of the sun, somewhat diminishing its rapidity. But, in conformity with a principle of mechanics well known to astronomers, though not familiar to the general reader, all the motion of rotation thus lost by the sun is added to the planets in the shape of annual motion of revolution, and thus their orbits all tend to enlarge,—they all tend to recede somewhat from the sun. But this state of things, though long-enduring enough, is after all only temporary, and will at any rate come to an end when the sun and planets have become solid. Meanwhile another set of circumstances is all the time tending to bring the planets nearer to the sun, and in the long run must gain the mastery. The space through which the planets move is filled with a kind of matter which serves as a medium for the transmission of heat and light, and this kind of matter, though different in some respects from ordinary ponderable matter, is yet like it in exerting friction. This friction is almost infinitely little, yet it has a wellnigh infinite length of time to work in, and during all this wellnigh infinite length of time it is slowly eating up the momentum of the planets and diminishing their ability to maintain their distances from the sun. Hence in course of time the planets will all fall into the sun, one after another, so that the solar system will end, as it began, by consisting of a single mass of matter.

But this is by no means the end of the story. When two bodies rush together, each parts with some of its energy of motion, and this lost energy of motion reappears as heat. In the concussion of two cosmical bodies, like the sun and the earth, an enormous quantity of motion is thus converted into heat. Now heat, when not allowed to radiate, or when generated faster than it can be radiated, is transformed into motion of expansion. Hence the shock of sun and planet would at once result in the vaporization of both bodies; and there can be no doubt that by the time the sun has absorbed the outermost of his attendant planets, he will have resumed something like his original nebulous condition. He will have been dilated into a huge mass of vapour, and will have become fit for a new process of contraction and for a new production of life-bearing planets.

We are now, however, confronted by an interesting but difficult question. Throughout all this grand past and future career of the solar system which we have just briefly traced, we have been witnessing a most prodigal dissipation of energy in the shape of radiant heat. At the outset we had an enormous quantity of what is called "energy of position," that is, the outer parts of our primitive nebula had a very long distance through which to travel towards one another in the slow process of concentration; and this distance was the measure of the quantity of work possible to our system. As the particles of our nebula drew nearer and nearer together, the energy of position continually lost reappeared continually as heat, of which the greater part was radiated off, but of which a certain amount was retained. All the gigantic amount of work achieved in the geologic development of our earth and its companion planets, and in the development of life wherever life may exist in our system, has been the product of this retained heat. At the present day the same wasteful process is going on. Each moment the sun's particles are losing energy of position as they draw closer and closer together, and the heat into which this lost energy is metamorphosed is poured out most prodigally in every direction. Let us consider for a moment how little of it gets used in our system. The earth's orbit is a nearly circular figure more than five hundred million miles in circumference, while only eight thousand miles of this path are at any one time occupied by the earth's mass. Through these eight thousand miles the sun's radiated energy is doing work, but through the remainder of the five hundred million it is idle and wasted. But the case is far more striking when we reflect that it is not in the plane of the earth's orbit only that the sun's radiance is being poured out. It is not an affair of a circle, but of a sphere. In order to utilize all the solar rays, we should need to have an immense number of earths arranged so as to touch each other, forming a hollow sphere around the sun, with the present radius of the earth's orbit. We may well believe Professor Tyndall, therefore, when he tells us that all the solar radiance we receive is less than a two-billionth part of what is sent flying through the desert regions of space. Some of the immense residue of course hits other planets stationed in the way of it, and is utilized upon their surfaces; but the planets, all put together, stop so little of the total quantity that our startling illustration is not materially altered by taking them into the account. Now this two-billionth part of the solar radiance poured out from moment to moment suffices to blow every wind, to raise every cloud, to drive every engine, to build up the tissue of every plant, to sustain the activity of every animal, including man, upon the surface of our vast and stately globe. Considering the wondrous richness and variety of the terrestrial life wrought out by the few sunbeams which we catch in our career through space, we may well pause overwhelmed and stupefied at the thought of the incalculable possibilities of existence which are thrown away with the potent actinism that darts unceasingly into the unfathomed abysms of immensity. Where it goes to or what becomes of it, no one of us can surmise.

Now when, in the remote future, our sun is reduced to vapour by the impact of the several planets upon his surface, the resulting nebulous mass must be a very insignificant affair compared with the nebulous mass with which we started. In order to make a second nebula equal in size and potential energy to the first one, all the energy of position at first existing should have been retained in some form or other. But nearly all of it has been lost, and only an insignificant fraction remains with which to endow a new system. In order to reproduce, in future ages, anything like that cosmical development which is now going on in the solar system, aid must be sought from without. We must endeavour to frame some valid hypothesis as to the relation of our solar system to other systems.

Thus far our view has been confined to the career of a single star,—our sun,—with the tiny, easily-cooling balls which it has cast off in the course of its development. Thus far, too, our inferences have been very secure, for we have been dealing with a circumscribed group of phenomena, the beginning and end of which have been brought pretty well within the compass of our imagination. It is quite another thing to deal with the actual or probable career of the stars in general, inasmuch as we do not even know how many stars there are, which form parts of a common system, or what are their precise dynamic relations to one another. Nevertheless we have knowledge of a few facts which may support some cautious inferences. All the stars which we can see are undoubtedly bound together by relations of gravitation. No doubt our sun attracts all the other stars within our ken, and is reciprocally attracted by them. The stars, too, lie mostly in or around one great plane, as is the case with the members of the solar system. Moreover, the stars are shown by the spectroscope to consist of chemical elements identical with those which are found in the solar system. Such facts as these make it probable that the career of other stars, when adequately inquired into, would be found to be like that of our own sun. Observation daily enhances this probability, for our study of the sidereal universe is continually showing us stars in all stages of development. We find irregular nebulae, for example; we find spiral and spheroidal nebulae; we find stars which have got beyond the nebulous stage, but are still at a whiter heat than our sun; and we also find many stars which yield the same sort of spectrum as our sun. The inference seems forced upon us that the same process of concentration which has gone on in the case of our solar nebula has been going on in the case of other nebulae. The history of the sun is but a type of the history of stars in general. And when we consider that all other visible stars and nebulae are cooling and contracting bodies, like our sun, to what other conclusion could we very well come? When we look at Sirius, for instance, we do not see him surrounded by planets, for at such a distance no planet could be visible, even Sirius himself, though fourteen times larger than our sun, appearing only as a "twinkling little star." But a comparative survey of the heavens assures us that Sirius can hardly have arrived at his present stage of concentration without detaching, planet-forming rings, for there is no reason for supposing that mechanical laws out there are at all different from what they are in our own system. And the same kind of inference must apply to all the matured stars which we see in the heavens.

When we duly take all these things into the account, the case of our solar system will appear as only one of a thousand cases of evolution and dissolution with which the heavens furnish us. Other stars, like our sun, have undoubtedly started as vaporous masses, and have thrown off planets in contracting. The inference may seem a bold one, but it after all involves no other assumption than that of the continuity of natural phenomena. It is not likely, therefore, that the solar system will forever be left to itself. Stars which strongly gravitate toward each other, while moving through a perennially resisting medium, must in time be drawn together. The collision of our extinct sun with one of the Pleiades, after this manner, would very likely suffice to generate even a grander nebula than the one with which we started. Possibly the entire galactic system may, in an inconceivably remote future, remodel itself in this way; and possibly the nebula from which our own group of planets has been formed may have owed its origin to the disintegration of systems which had accomplished their career in the depths of the bygone eternity.

When the problem is extended to these huge dimensions, the prospect of an ultimate cessation of cosmical work is indefinitely postponed, but at the same time it becomes impossible for us to deal very securely with the questions we have raised. The magnitudes and periods we have introduced are so nearly infinite as to baffle speculation itself: One point, however, we seem dimly to discern. Supposing the stellar universe not to be absolutely infinite in extent, we may hold that the day of doom, so often postponed, must come at last. The concentration of matter and dissipation of energy, so often checked, must in the end prevail, so that, as the final outcome of things, the entire universe will be reduced to a single enormous ball, dead and frozen, solid and black, its potential energy of motion having been all transformed into heat and radiated away. Such a conclusion has been suggested by Sir William Thomson, and it is quite forcibly stated by the authors of "The Unseen Universe." They remind us that "if there be any one form of energy less readily or less completely transformable than the others, and if transformations constantly go on, more and more of the whole energy of the universe will inevitably sink into this lower grade as time advances." Now radiant heat, as we have seen, is such a lower grade of energy. "At each transformation of heat-energy into work, a large portion is degraded, while only a small portion is transformed into work. So that while it is very easy to change all of our mechanical or useful energy into heat, it is only possible to transform a portion of this heat-energy back again into work. After each change, too, the heat becomes more and more dissipated or degraded, and less and less available for any future transformation. In other words," our authors continue, "the tendency of heat is towards equalization; heat is par excellence the communist of our universe, and it will no doubt ultimately bring the system to an end..... It is absolutely certain that life, so far as it is physical, depends essentially upon transformations of energy; it is also absolutely certain that age after age the possibility of such transformations is becoming less and less; and, so far as we yet know, the final state of the present universe must be an aggregation (into one mass) of all the matter it contains, i. e. the potential energy gone, and a practically useless state of kinetic energy, i. e. uniform temperature throughout that mass." Thus our authors conclude that the visible universe began in time and will in time come to an end; and they add that under the physical conditions of such a universe "immortality is impossible."

Concerning the latter inference we shall by and by have something to say. Meanwhile this whole speculation as to the final cessation of cosmical work seems to me—as it does to my friend, Professor Clifford 3—by no means trustworthy. The conditions of the problem so far transcend our grasp that any such speculation must remain an unverifiable guess. I do not go with Professor Clifford in doubting whether the laws of mechanics are absolutely the same throughout eternity; I cannot quite reconcile such a doubt with faith in the principle of continuity. But it does seem to me needful, before we conclude that radiated energy is absolutely and forever wasted, that we should find out what becomes of it. What we call radiant heat is simply transverse wave-motion, propagated with enormous velocity through an ocean of subtle ethereal matter which bathes the atoms of all visible or palpable bodies and fills the whole of space, extending beyond the remotest star which the telescope can reach. Whether there are any bounds at all to this ethereal ocean, or whether it is as infinite as space itself, we cannot surmise. If it be limited, the possible dispersion of radiant energy is limited by its extent. Heat and light cannot travel through emptiness. If the ether is bounded by surrounding emptiness, then a ray of heat, on arriving at this limiting emptiness, would be reflected back as surely as a ball is sent back when thrown against a solid wall. If this be the case, it will not affect our conclusions concerning such a tiny region of space as is occupied by the solar system, but it will seriously modify Sir William Thomson's suggestion as to the fate of the universe as a whole. The radiance thrown away by the sun is indeed lost so far as the future of our system is concerned, but not a single unit of it is lost from the universe. Sooner or later, reflected back in all directions, it must do work in one quarter or another, so that ultimate stagnation be comes impossible. It is true that no such return of radiant energy has been detected in our corner of the world; but we have not yet so far disentangled all the force-relations of the universe that we are entitled to regard such a return as impossible. This is one way of escape from the consummation of things depicted by our authors. Another way of escape is equally available, if we suppose that while the ether is without bounds the stellar universe also extends to infinity. For in this case the reproduction of nebulous masses fit for generating new systems of worlds must go on through space that is endless, and consequently the process can never come to an end and can never have had a beginning. We have, therefore, three alternatives: either the visible universe is finite, while the ether is infinite; or both are finite; or both are infinite. Only on the first supposition, I think, do we get a universe which began in time and must end in time. Between such stupendous alternatives we have no grounds for choosing. But it would seem that the third, whether strictly true or not, best represents the state of the case relatively to our feeble capacity of comprehension. Whether absolutely infinite or not, the dimensions of the universe must be taken as practically infinite, so far as human thought is concerned. They immeasurably transcend the capabilities of any gauge we can bring to bear on them. Accordingly all that we are really entitled to hold, as the outcome of sound speculation, is the conception of innumerable systems of worlds concentrating out of nebulous masses, and then rushing together and dissolving into similar masses, as bubbles unite and break up—now here, now there—in their play on the surface of a pool, and to this tremendous series of events we can assign neither a beginning nor an end.

We must now make some more explicit mention of the ether which carries through space the rays of heat and light. In closest connection with the visible stellar universe, the vicissitudes of which we have briefly traced, the all-pervading ether constitutes a sort of unseen world remarkable enough from any point of view, but to which the theory of our authors ascribes capacities hitherto unsuspected by science. The very existence of an ocean of ether enveloping the molecules of material bodies has been doubted or denied by many eminent physicists, though of course none have called in question the necessity for some interstellar medium for the transmission of thermal and luminous vibrations. This scepticism has been, I think, partially justified by the many difficulties encompassing the conception, into which, however, we need not here enter. That light and heat cannot be conveyed by any of the ordinary sensible forms of matter is unquestionable. None of the forms of sensible matter can be imagined sufficiently elastic to propagate wave-motion at the rate of one hundred and eighty-eight thousand miles per second. Yet a ray of light is a series of waves, and implies some substance in which the waves occur. The substance required is one which seems to possess strangely contradictory properties. It is commonly regarded as an "ether" or infinitely rare substance; but, as Professor Jevons observes, we might as well regard it as an infinitely solid "adamant." "Sir John Herschel has calculated the amount of force which may be supposed, according to the undulatory theory of light, to be exerted at each point in space, and finds it to be 1,148,000,000,000 times the elastic force of ordinary air at the earth's surface, so that the pressure of the ether upon a square inch of surface must be about 17,000,000,000,000, or seventeen billions of pounds." 4 Yet at the same time the resistance offered by the ether to the planetary motions is too minute to be appreciable. "All our ordinary notions," says Professor Jevons, "must be laid aside in contemplating such an hypothesis; yet [it is] no more than the observed phenomena of light and heat force us to accept. We cannot deny even the strange suggestion of Dr. Young, that there may be independent worlds, some possibly existing in different parts of space, but others perhaps pervading each other, unseen and unknown, in the same space. For if we are bound to admit the conception of this adamantine firmament, it is equally easy to admit a plurality of such."

The ether, therefore, is unlike any of the forms of matter which we can weigh and measure. In some respects it resembles a fluid, in some respects a solid. It is both hard and elastic to an almost inconceivable degree. It fills all material bodies like a sea in which the atoms of the material bodies are as islands, and it occupies the whole of what we call empty space. It is so sensitive that a disturbance in any part of it causes a "tremour which is felt on the surface of countless worlds." Our old experiences of matter give us no account of any substance like this; yet the undulatory theory of light obliges us to admit such a substance, and that theory is as well established as the theory of gravitation. Obviously we have here an enlargement of our experience of matter. The analysis of the phenomena of light and radiant heat has brought us into mental relations with matter in a different state from any in which we previously knew it. For the supposition that the ether may be something essentially different from matter is contradicted by all the terms we have used in describing it. Strange and contradictory as its properties may seem, are they any more strange than the properties of a gas would seem if we were for the first time to discover a gas after heretofore knowing nothing but solids and liquids? I think not; and the conclusion implied by our authors seems to me eminently probable, that in the so-called ether we have simply a state of matter more primitive than what we know as the gaseous state. Indeed, the conceptions of matter now current, and inherited from barbarous ages, are likely enough to be crude in the extreme. It is not strange that the study of such subtle agencies as heat and light should oblige us to modify them; and it will not be strange if the study of electricity should entail still further revision of our ideas.

We are now brought to one of the profoundest speculations of modern times, the vortex-atom theory of Helmholtz and Thomson, in which the evolution of ordinary matter from ether is plainly indicated. The reader first needs to know what vortex-motion is; and this has been so beautifully explained by Professor Clifford, that I quote his description entire: "Imagine a ring of india-rubber, made by joining together the ends of a cylindrical piece (like a lead-pencil before it is cut), to be put upon a round stick which it will just fit with a little stretching. Let the stick be now pulled through the ring while the latter is kept in its place by being pulled the other way on the outside. The india-rubber has then what is called vortex-motion. Before the ends were joined together, while it was straight, it might have been made to turn around without changing position, by rolling it between the hands. Just the same motion of rotation it has on the stick, only that the ends are now joined together. All the inside surface of the ring is going one way, namely, the way the stick is pulled; and all the outside is going the other way. Such a vortex-ring is made by the smoker who purses his lips into a round hole and sends out a puff of smoke. The outside of the ring is kept back by the friction of his lips while the inside is going forwards; thus a rotation is set up all round the smoke-ring as it travels out into the air." In these cases, and in others as we commonly find it, vortex-motion owes its origin to friction and is after a while brought to an end by friction. But in 1858 the equations of motion of an incompressible frictionless fluid were first successfully solved by Helmholtz, and among other things he proved that, though vortex-motion could not be originated in such a fluid, yet supposing it once to exist, it would exist to all eternity and could not be diminished by any mechanical action whatever. A vortex-ring, for example, in such a fluid, would forever preserve its own rotation, and would thus forever retain its peculiar individuality, being, as it were, marked off from its neighbour vortex-rings. Upon this mechanical truth Sir William Thomson based his wonderfully suggestive theory of the constitution of matter. That which is permanent or indestructible in matter is the ultimate homogeneous atom; and this is probably all that is permanent, since chemists now almost unanimously hold that so-called elementary molecules are not really simple, but owe their sensible differences to the various groupings of an ultimate atom which is alike for all. Relatively to our powers of comprehension the atom endures eternally; that is, it retains forever unalterable its definite mass and its definite rate of vibration. Now this is just what a vortex-ring would do in an incompressible frictionless fluid. Thus the startling question is suggested, Why may not the ultimate atoms of matter be vortex-rings forever existing in such a frictionless fluid filling the whole of space? Such a hypothesis is not less brilliant than Huyghens's conjectural identification of light with undulatory motion; and it is moreover a legitimate hypothesis, since it can be brought to the test of verification. Sir William Thomson has shown that it explains a great many of the physical properties of matter: it remains to be seen whether it can explain them all.

Of course the ether which conveys thermal and luminous undulations is not the frictionless fluid postulated by Sir William Thomson. The most conspicuous property of the ether is its enormous elasticity, a property which we should not find in a frictionless fluid. "To account for such elasticity," says Professor Clifford (whose exposition of the subject is still more lucid than that of our authors), "it has to be supposed that even where there are no material molecules the universal fluid is full of vortex-motion, but that the vortices are smaller and more closely packed than those of [ordinary] matter, forming altogether a more finely grained structure. So that the difference between matter and ether is reduced to a mere difference in the size and arrangement of the component vortex-rings. Now, whatever may turn out to be the ultimate nature of the ether and of molecules, we know that to some extent at least they obey the same dynamic laws, and that they act upon one another in accordance with these laws. Until, therefore, it is absolutely disproved, it must remain the simplest and most probable assumption that they are finally made of the same stuff, that the material molecule is some kind of knot or coagulation of ether." 5

Another interesting consequence of Sir William Thomson's pregnant hypothesis is that the absolute hardness which has been attributed to material atoms from the time of Lucretius downward may be dispensed with. Somewhat in the same way that a loosely suspended chain becomes rigid with rapid rotation, the hardness and elasticity of the vortex-atom are explained as due to the swift rotary motion of a soft and yielding fluid. So that the vortex-atom is really indivisible, not by reason of its hardness or solidity, but by reason of the indestructibleness of its motion.

Supposing, now, that we adopt provisionally the vortex theory,—the great power of which is well shown by the consideration just mentioned,—we must not forget that it is absolutely essential to the indestructibleness of the material atom that the universal fluid in which it has an existence as a vortex-ring should be entirely destitute of friction. Once admit even the most infinitesimal amount of friction, while retaining the conception of vortex-motion in a universal fluid, and the whole case is so far altered that the material atom can no longer be regarded as absolutely indestructible, but only as indefinitely enduring. It may have been generated, in bygone eternity, by a natural process of evolution, and in future eternity may come to an end. Relatively to our powers of comprehension the practical difference is perhaps not great. Scientifically speaking, Helmholtz and Thomson are as well entitled to reason upon the assumption of a perfectly frictionless fluid as geometers in general are entitled to assume perfect lines without breadth and perfect surfaces without thickness. Perfect lines and surfaces do not exist within the region of our experience; yet the conclusions of geometry are none the less true ideally, though in any particular concrete instance they are only approximately realized. Just so with the conception of a frictionless fluid. So far as experience goes, such a thing has no more real existence than a line without breadth; and hence an atomic theory based upon such an assumption may be as true ideally as any of the theorems of Euclid, but it can give only an approximatively true account of the actual universe. These considerations do not at all affect the scientific value of the theory; but they will modify the tenour of such transcendental inferences as may be drawn from it regarding, the probable origin and destiny of the universe.

The conclusions reached in the first part of this paper, while we were dealing only with gross visible matter, may have seemed bold enough; but they are far surpassed by the inference which our authors draw from the vortex theory as they interpret it. Our authors exhibit various reasons, more or less sound, for attributing to the primordial fluid some slight amount of friction; and in support of this view they adduce Le Sage's explanation of gravitation as a differential result of pressure, and Struve's theory of the partial absorption of light-rays by the ether,—questions with which our present purpose does not require us to meddle. Apart from such questions it is every way probable that the primary assumption of Helmholtz and Thomson is only an approximation to the truth. But if we accredit the primordial fluid with even an infinitesimal amount of friction, then we are required to conceive of the visible universe as developed from the invisible and as destined to return into the invisible. The vortex-atom, produced by infinitesimal friction operating through wellnigh infinite time, is to be ultimately abolished by the agency which produced it. In the words of our authors, "If the visible universe be developed from an invisible which is not a perfect fluid, then the argument deduced by Sir William Thomson in favour of the eternity of ordinary matter disappears, since this eternity depends upon the perfect fluidity of the invisible. In fine, if we suppose the material universe to be composed of a series of vortex-rings developed from an invisible universe which is not a perfect fluid, it will be ephemeral, just as the smoke-ring which we develop from air, or that which we develop from water, is ephemeral, the only difference being in duration, these lasting only for a few seconds, and the others it may be for billions of years." Thus, as our authors suppose that "the available energy of the visible universe will ultimately be appropriated by the invisible," they go on to imagine, "at least as a possibility, that the separate existence of the visible universe will share the same fate, so that we shall have no huge, useless, inert mass existing in after ages to remind the passer-by of a form of energy and a species of matter that is long since out of date and functionally effete. Why should not the universe bury its dead out of sight?"

In one respect perhaps no more stupendous subject of contemplation than this has ever been offered to the mind of man. In comparison with the length of time thus required to efface the tiny individual atom, the entire cosmical career of our solar system, or even that of the whole starry galaxy, shrinks into utter nothingness. Whether we shall adopt the conclusion suggested must depend on the extent of our speculative audacity. We have seen wherein its probability consists, but in reasoning upon such a scale we may fitly be cautious and modest in accepting inferences, and our authors, we may be sure, would be the first to recommend such modesty and caution. Even at the dimensions to which our theorizing has here grown, we may for instance discern the possible alternative of a simultaneous or rhythmically successive generation and destruction of vortex-atoms which would go far to modify the conclusion just suggested. But here we must pause for a moment, reserving for a second paper the weightier thoughts as to futurity which our authors have sought to enwrap in these sublime physical speculations.