# In the High Heavens/Chapter 11

CHAPTER XI.
HOW LONG CAN THE EARTH SUSTAIN LIFE?
T seems to be worth while to collect together what may be said on the subject of the duration of life on the globe viewed as a problem in physics, and this is the subject I propose to discuss in the present chapter. In the first place, it will be desirable to define a little more clearly the exact question which is to engage us, so as to avoid raising collateral inquiries on which it would not be convenient now to enter. Let it be first of all understood that I am not intending to discuss at present the question from its biological point of view, at least not more than to allude to the conceivability that there can be biological reasons for anticipating a termination to man's existence at some time or other. Why, it may be asked, should the human species expect to enjoy perennial existence, seeing that the facts of palaeontology show us that multitudes of races of animals have had their little day, and vanished? It would, at least, be necessary for man to see clear grounds for his belief before he could fancy himself entitled to an immunity from the destruction which seems to be the destiny of other species. Biological agents for the extinction of man have been suggested with plausibility. The influenza bacillus was ​lately rampant over the world. Is there any security against some other bacillus quite as ubiquitous, and ten times as fatal, coming to take its abode among us? It may be that the intelligence of man shall be able to cope with the deadly influences that are around him, and that thus the human race may be preserved from the annihilation that seems to await all unintelligent races of animals. The Pasteurs of the future may be able to devise means by which the ravages of the bacilli in the human body can be still further restrained, even if not wholly frustrated. The advent of intelligent beings on the globe has certainly introduced a factor into evolution the full import of which we are not at present able to appreciate.
Speaking broadly, we may assert that every species of animal gradually vanishes, or is transformed into what may be considered a creation of different character. There are, of course, a few apparent exceptions among organizations of a low type. But the instances of such identities are comparatively few, and they are not to be met with among animals of the higher type. Though some of the lower animals to which we have referred may be of more abiding duration than the higher forms, yet it by no means follows that any of the lower types are qualified for indefinitely long existence. It seems much more likely that, when sufficient time has elapsed, they will not be found exceptions to the law that the duration of every species is limited. The palaeontological evidence, so far as it goes, must therefore be held to suggest that the present human animal, like every other species, is necessarily doomed to disappear, unless in so far as the presence of intelligence may be able to avert the fate that seems to attend every species in which intelligence is absent.
​How far intelligence may be able to accomplish this is a point on which palaeontology gives no guidance whatever. Would the plesiosaurus, if he had been gifted with reasoning power, have been able to do such battle for his race that they would have survived those changes and chances which have certainly swept such creatures from existence P Without speculating on such a question, we may, nevertheless, believe that intelligence can sometimes confer on the species which possesses it a degree of pliancy in accommodating itself to altered conditions of the environment superior to that enjoyed by organisms without intellectual power. It may be noted that man has preserved at least one species of animal from the extinction which to all appearance would otherwise have overtaken it. The camel, as a wild animal, is said to be now extinct. Its nearest ally at present living in a state of nature must be sought in the New World. The camel itself and its immediate congeners have apparently been so extirpated as wild animals, that it is to the llamas and alpacas of Peru that we have to look for the nearest wild, animals to the ship of the desert, which has from time immemorial been domesticated in the East. It is at least conceivable that what man has been able to do for other races of animals he can also do on behalf of that race to which he himself belongs.
Suppose that the succession of summer and winter, of seedtime and harvest, were to last indefinitely; suppose that the sun were never to be less generous in the dispensing of his benefits than he is at present, it is quite possible that man's intelligence might be able to defeat various enemies which threaten the extinction of his species. It seems useless for us to discuss this question, ​for it is perfectly certain that though man might successfully combat some of the agents seeking for his destruction, there is one that it would be wholly beyond his power to subdue. An agent over which he has and can have no control whatever imposes a term to his existence; nor does it seem possible for human intelligence to avert the threatened doom. To point out the necessity for this conclusion is my object in this chapter.
I know that in the present day there are many who seem to think that hardly any boundaries can be assigned to the resources of a reasoning being. I have heard that when King Hudson in the zenith of his fame was asked as to what his railways were to do when all the coal was burned out, he replied that by that time we should have learned how to burn water. Those who are asked the same question now, will often reply that they will use electricity, and doubtless think that they have thus disposed of the question. The fallacy of such answers is obvious. A so-called "water-gas" may no doubt be used for developing heat, but it is not the water which supplies the energy. Trains may be run by electricity, but all that the electricity does is to convey the energy from the point where it is generated to the train which is in motion. Electricity is itself no more a source of power than is the rope with which a horse drags a boat along the canal. There is much more philosophy in the old saying, "Money makes the mare to go," than in the optimistic doctrine we often hear spoken of with regard to the capacity of man for dealing with nature.
The fact is that a very large part of the boasted advance of civilisation is merely the acquisition of an increased capability of squandering. For what are we doing every ​day but devising fresh appliances to exhaust with ever greater rapidity the hoard of coal? There is just a certain number of tons of coal lying in the earth, and when these are gone there can be no more forthcoming. There is no manufacture of coal in progress at the present time. The useful mineral was the product of a very singular period in the earth's history, the like of which has not again occurred in any noteworthy degree in the geological ages that have since run their course. Our steam-engines are methods of spending this hoard; and what we often hear lauded as some triumph in human progress is merely the development of some fresh departure in a frightful extravagance. We would justly regard a man as guilty of expending his substance wastefully if he could not perform a journey without a coach-and-six and half-a-dozen out-riders, and yet we insist that the great steamers which take us across the Atlantic shall be run at a speed which requires engines, let us say, of 12,000 horse-power. If the number of passengers on such a vessel be set down as 500, we have for each passenger the united force of 24 horses, night and day, throughout the voyage. I expect our descendants will think that our coal-cellars have been emptied in a very wasteful manner, particularly when they reflect that if we had been content with a speed somewhat less than that at present demanded the necessary consumption of coal would have been reduced in a far greater proportion than the mere alteration of speed would imply.
Of course, no one will contend that the exhaustion of coal means the end of the human race; man lived here for tens of thousands of years before he learned how to use coal. There may be a sort of Chinese-like ​civilisation quite compatible with the absence of mineral fuel, at all events in regions where the climate is tolerably mild. We must also remember, as has often been pointed out, that there are vast stores of energy available elsewhere. The radiation from the sun, if it could be suitably garnered up and employed both directly as heat and indirectly as a source of power, would be quite capable of supplying all conceivable wants of humanity for ages. It is also to be noted that we live on the outside of a globe the inside of which is filled with substances that appear, from all we can learn, to have a temperature not less than that of molten iron. If the crust could be pierced sufficiently far, vast indeed is the quantity of heat that might be available. We see the operation of tapping the internal heat going on in nature. Every volcanic outbreak, every spring of hot water, every geyser are but indications of the internal heat of our globe. It may indeed be hard to see how a practical method for drawing on this vast reserve of heat can be devised, but it is at least conceivable that it may be rendered available when the coal and other more accessible sources have become exhausted, or even when their yield has considerably lessened.
The coal of England may last a century or two; the coal in other parts of the globe may supply our cellars for a few centuries more, but the exhaustion of this truly marvellous product is proceeding at an accelerated pace. Doubtless the end of the coal, at least as an article of a mighty commerce, will arrive within a period brief in comparison with the ages of human existence. In the history of humanity from first to last the few centuries through which we are now passing will stand out prominently as the coal-burning period.
​It is a noteworthy fact that the possibility of the continued existence of the human race depends fundamentally upon the question of heat. If heat, or what is equivalent to heat, does not last, then man cannot last either. There is no shirking this plain truism. It is therefore necessary to review carefully the possible sources of heat and see how far they can be relied upon to provide a continuous supply.
Of course it is obvious that the available heat generally comes from the sun. It may be used directly, or it may be and often is used indirectly, for nothing can be more certain than that it is sun heat in a modified form which radiates from a coal fire in the drawing-room or from a log fire in the backwoods. As the sun shines on the growing vegetation, the leaves extract the warmth from the sunbeams. The organism wants carbon, and to obtain it decomposes the carbonic acid gas of which a certain proportion is always present in the air. To decompose this gas requires the expenditure of heat or of what is equivalent to heat. But this does not show itself in raising the temperature of the carbon and oxygen after they have been dissociated. Their temperature may be no higher than was that of the carbonic acid from which they have come, but the heat has been expended in the process of forcing the several molecules asunder from the close and intimate union of their combined condition.
As the growing plant must have carbon, it draws that carbon from the atmosphere, and the heat that is required to effect the decomposition of the carbonic acid is obtained from sunbeams. When the carbon thus derived by the plant comes ultimately to be burned it reunites with the oxygen of the air, and in the act of doing so evolves an ​amount of heat precisely equivalent to that which was absorbed from the sunbeams. Thus it is that the heat now radiating from our fireplaces has at some time previously been transmitted to the earth from the sun. If it be timber that we are burning, then we are using the sunbeams that have shone on the earth within a few decades. If it be coal, then we are retransforming to heat the solar energy which arrived at the earth millions of years ago.
The question as to the continued existence of man on this globe resolves itself eventually into an investigation as to the permanence of the heat- supply. Doubtless human life requires many other conditions, but of this we may feel assured, that if the heat fail and if nothing else be forthcoming which can be transformed into heat, then most assuredly from this cause alone there is a term to human existence. Before discussing the prospect of the duration of sunbeams we may first consider a few other less important sources of heat. So far as the coal goes, we have already observed that as it is limited in quantity it can offer no perennial supply. Doubtless there is in the earth some quantity of other materials capable of oxidation, or of undergoing other chemical change; in the course of which, and as an incident of such change, heat is evolved. The amount of heat that can possibly arise from such sources is strictly limited. There is in the entire earth just a certain number of units of heat possible from such chemical combinations, but after the combination has been effected there cannot be any more heat from this source.
Then as to the internal heat of the earth due to the incandescent state of its interior. Here there is no doubt ​a large store of energy, but still it is of limited quantity, and it is also on the wane. This heat is occasionally copiously liberated by volcanoes, but ordinarily the transit of heat from the interior to the surface and its discharge from thence by radiation is a slow process. It is, however, sufficient for our present purpose to observe that slow though the escape may be, it is incessantly going on. There is only a definite number of units of heat contained in the interior of the earth at this moment, and as they are gradually diminishing, and as there is no source from which the loss can be replenished, there is here no supply of warmth that can be relied on permanently. It must also be mentioned that there exists another store of energy which under certain conditions admits of being transformed into heat. I allude to the energy which the earth possesses in virtue of its rapid rotation on its axis.
In this respect we may liken our globe to a mighty flywheel which contains a certain quantity of energy that must be poured forth as its speed is reduced. It is the action of the tides which enables this form of earth energy to be transformed into heat. The tides check the speed with which the earth rotates. The energy thus lost must in part at least be transformed into heat, which is then again lost by radiation into space. Of course the quantity of energy which the earth possesses by reason of its rotation is of limited amount, and it is steadily being dissipated, just as the internal heat is being lost and just as the potential heat that exists in consequence of unsatisfied chemical attraction is also declining. It seems that whenever the tides shall have so checked the earth that it only rotates at half its present speed, the quantity ​of the energy now existing in consequence of the rotation will have been reduced to a fourth of its present value.
Next as to the various forms in which sun-heat is received. We have already referred to the mode in which it is captured by growing plants. There is also another indirect method in which the sun-heat is made to provide energy useful to man. The waterfall which turns the mill-wheel is of course really efficient because the water is running down, and it can only run down because it has first been raised up. This raising is accomplished by sunbeams. They beat down on the wide expanse of the great oceans, and the vapour thence arising soars aloft into the heights of the atmosphere where it forms clouds. It is of course the solar energy that has performed this task of lifting, and as the rain descends it becomes collected into the streams and rivers which on their way to the sea are made to turn the waterwheels. In like manner it is of course the action of the sun which sets in motion great volumes of air to form the winds, so that when we employ windmills to grind our corn we are utilising energy diffused from the sun.
It goes without saying that the welfare of the human race is necessarily connected with the continuance of the sun's beneficent action. We have indeed shown that the few other direct or indirect sources of heat which might conceivably be relied upon are in the very nature of things devoid of the necessary permanence. It becomes therefore of the utmost interest to inquire whether the sun's heat can be calculated on indefinitely. Here is indeed a subject which is literally of the most vital importance so far as organic life is concerned. If the sun shall ever cease to shine, then must it be certain that there is a term ​beyond which human existence, or indeed, organic existence of any type whatever, cannot any longer endure on the earth.
We may say once for all that the sun contains just a certain number of units of heat actual or potential, and that he is at the present moment shedding that heat around with the most appalling extravagance. No doubt the heat-hoard of the sun is so tremendous that the consequences of his mighty profusion do not become speedily apparent. They are indeed, it must be admitted, hardly to be discerned within the few brief centuries that the sun has been submitted to human observation. But we have grounds for knowing as a certainty that the sun cannot escape from the destiny that sooner or later overtakes the spendthrift. In his interesting studies of this subject, Professor Langley gives a striking illustration of the rate at which the solar heat is being squandered at this moment.
He remarks that the great coalfields of Pennsylvania contain enough of the precious mineral to supply the wants of the United States for a thousand years. If all that tremendous accumulation of fuel were to be extracted and burned in one vast conflagration, the total quantity of heat that would be produced would no doubt be stupendous, and yet, says this authority, who has taught us so much about the sun, all the heat developed by that terrific coal fire would not be equal to that which the sun pours forth in the thousandth part of each single second. When we reflect that this expenditure of heat has been going on not alone for the centuries during which the earth has been the abode of man, but also for those periods which we cannot estimate, except by saying ​that they are doubtless millions of years, during which there has been life on the globe, then indeed we begin to comprehend how vast must have been the capital of heat with which the sun started on its career.
But now for the question, of supreme importance so far as organic life is concerned, as to the possibility of the indefinite duration of the sun as a source of radiant energy. It may indeed be urged that there is no apparent decline in the warmth of the sun and the brilliancy of the light that it diffuses. There is no reason to think from any historical evidence, or indeed from any evidence whatever, that there is the slightest measurable difference between the radiance of the sun that was shed on the inhabitants of ancient Greece and the radiance that still falls on the same classic soil. So far as our knowledge goes, the plants that now grow on the hills and plains of Greece are the same as the plants which grew on the same hills and plains two thousand years ago. It is, of course, true that the significance of the argument is affected by the circumstance that organisms by the influence of natural selection can preserve a continuous adaptation to an environment which is gradually becoming modified. The olive grows in Greece now, and a tree called by the same name grew there a couple of thousand years ago. I do not suppose that anyone is likely to doubt that the ancient olive and the modern olive are at all events so far alike that plants identical in every respect with the olive of ancient times could flourish where the modern olive now abounds. That there have been great climatic vicissitudes in times past is of course clearly shown by the records of the rocks. It is almost certain that astronomical causes have been largely concerned in ​the production of these changes, but from among these causes we may exclude the variations in the sun's heat. There does not seem to be the least reason to suppose that any alteration in the rate at which the sun diffuses heat has been a cause of the vicissitudes of climates which the earth has certainly undergone within geological times.
And yet we feel certain that the incessant radiation from the sun must be producing a profound effect on its stores of energy. The only way of reconciling this with the total absence of evidence of the expected changes is to be found in the supposition that such is the mighty mass of the sun, such the prodigious supply of heat, or what is equivalent to heat, which it contains, that the grand transformation through which it is passing proceeds at a rate so slow that, during the ages accessible to our observations, the results achieved have been imperceptible. Think of a sphere the size of the earth. Would it be possible to detect the curvature of a portion of its equator a yard in length? To our senses, nay, even to our most refined measurements, such a line, though indeed a portion of a circular arc, would be indistinguishable from a straight line. So is it with the solar radiation. To our ephemeral glance it appears to be quite uniform; we can only study a very minute part of the whole series of changes, so that we are as little able to detect the want of uniformity as we should be to detect the departure from a straight line of the arc of a circle which we have given as an illustration.
We cannot, however, attribute to the sun any miraculous power of generating heat. That great body cannot disobey those laws which we have learned from ​experiments in our laboratories. Of course no one now doubts that the great law of the conservation of energy holds good. We do not in the least believe that because the sun's heat is radiated away in such profusion that it is therefore entirely lost. It travels off no doubt to the depths of space, and as to what may become of it there we have no information. Everything we know points to the law that energy is as indestructible as matter itself. The heat scattered from the sun exists at least as ethereal vibration if in no other form. But it is most assuredly true that this energy so copiously dispensed is lost to our solar system. There is no form in which it is returned, or in which it can be returned. The energy of the system is as surely declining as the store of energy of the clock declines according as the weight runs down. In the clock, however, the energy is restored by winding up the weight, but there is no analogous process known in our system.
It was long a mystery how the sun was able to retain its heat so as to supply continually its prodigious rate of expenditure. The suppositions that would most naturally occur were shown to be utterly insufficient. We know that a great iron casting often takes many hours to grow cold after it has been drawn from the mould. If the casting be a sufficiently large one, the cooling will proceed so slowly that it will not get cold for days, because the tardiness of cooling increases with the dimensions of the body. It was not, perhaps, unnatural to suppose that as the sun was so vast, the process of cooling would proceed with such extreme slowness that notwithstanding the quantity of heat poured out every second, the annual amount of loss would be so small relatively to the whole ​store that the effect of that loss would be imperceptible in such periods as those over which our knowledge extends. This supposition, however plausible, is speedily demolished when brought to the test by which all such questions must be decided—the test of actual calculation.
We can determine with all needful accuracy the store of heat that the sun would contain if regarded merely as a white-hot solid globe. When we apply the known annual loss, we see at once that if the sun had merely the simple constitution here supposed, the annual expenditure would bear such a considerable proportion to the total supply that the effect of the loss would become speedily apparent. It is certain that the sun must under such circumstances fall some degrees in temperature each year. In a couple of thousand years the change in temperature would be sufficiently great to affect in the profoundest manner the supply of sunbeams. As, however, we know that for a couple of thousand years, or, indeed, for periods much longer still, there has been no perceptible decrease in the volume of solar radiations, we conclude that the great luminary cannot be regarded merely as a glowing solid globe dispensing its heat by radiation.
There is another supposition as to the continuance of sun heat which must be mentioned, only, however, to be dismissed as quite incapable of offering any solution of the problem. As we generate heat here so largely by the combination of fuel, it has been sometimes thought that a similar process may be in progress on the sun. It has been supposed that elements capable and desirous of chemical union may exist in the sun in such profusion that by their entering into association a quantity of heat is liberated sufficient to account for the continuous dispersal by radiation. ​Here, again, the test must be applied which is decisive of such pretensions. It may certainly be the case that chemical actions of one kind or another are going on in the sun, and among them are doubtless some of such a character that they evolve heat. But we happen to know exactly how much heat can be evolved by the action of specified quantities of elementary bodies by whose union heat is generated. It appears clear from the figures that chemical action is a wholly inadequate method of accounting for solar radiation. To take one instance, we may mention that if the sun had been a globe of white-hot carbon, and if there had been a sufficient supply of oxygen to effect its combustion, the total heat generated by the entire mass would not supply the solar radiation for the period that has elapsed since the building of the pyramids. It is, therefore, clear that the supposition that the sun is a burning globe, like the supposition of the sun as a cooling solid globe, is quite inadequate to explain the marvellous persistence with which, for countless ages, the orb of day has distributed its beams.
There is another supposition which, though not itself providing the explanation that we are searching for, still points so far in that direction that I have kept it till the last. It has been sometimes suggested that the dashing of meteoric matter into the sun from outside may afford the requisite supply of energy. There can be no doubt that the plunge of a meteor into the sun's atmosphere with the terrific velocity which it will necessarily acquire in consequence of the attraction of the sun, is accompanied by the transformation of the energy of the meteor's movement into light and heat. The quantity of energy that a meteor thus carries with it is so vast that ​it is hardly credible until the figures which express it and the grounds on which they are based have received due attention. Let us think of a meteor which is moving, as such bodies do when near the earth, with a speed perhaps a hundred times as great as that of a bullet from a rifle, or even from one of the most finished pieces of artillery. The energy of the meteor, depending as it does upon the square of the velocity, will be, therefore, about ten thousand times that of the bullet of the same size. It seems that the energy thus possessed by a meteor one pound in weight is as much as could be developed by the explosion of a ton weight of gunpowder. Doubtless, in the vicinity of the sun, the meteors are more numerous, and they move with a vastly higher velocity than the meteors near the earth. It is therefore plain that the quantity of energy contributed to the sun from this source must be very large in amount. It can, however, be shown that there are not enough meteors in existence to supply a sufficient quantity of heat to the sun to compensate the loss by radiation. The indraught of meteoric matter may indeed certainly tend in some small degree to retard the ultimate cooling of the great luminary, but its effect is so small that we can quite afford to overlook it from the point of view that we are taking in these pages.
It is to Helmholtz we are indebted for the true solution of the long-vexed problem. He has demonstrated, in the clearest manner, where the source of the sun's heat lies. It depends upon a cause which, at the first glance, would seem an insignificant one, but which the arithmetical test, that is so essential, at once raises to a position of the greatest importance. It is sufficiently obvious that the sun is in no sense to be regarded as a ​solid body. It seems very unlikely that there can be throughout its entire extent any portion which possesses the properties of a solid; certainly those exterior parts of the sun which are all that are accessible to our observation are anything but solid: they are vast volumes of luminous material floating in gases of a much less luminous nature. The openings between the clouds form the spots, while the mighty projections which leap from the sun's surface testify in the most emphatic manner to the gaseous or vaporous character of the outer parts of the great luminary. A gaseous globe like the sun when it parts with its heat observes laws of a very different type from those which a cooling solid follows. As the heat disappears by radiation the body contracts; the gaseous object, however, decreases in general much more than a solid body would do for the same loss of heat. This is connected with a striking difference between the manner in which the two bodies change in temperature. The solid, as it loses heat, also loses temperature; the gas, on the other hand, does not necessarily lose temperature even though it is losing heat. Indeed, it may happen that the very fact that the gaseous globe is losing heat may be the cause of its actually gaining in temperature and becoming hotter. This seems a paradox at the first glance, but it will be found not to be so when due attention is paid to the different notions that belong to the words h?at and temperature. The globe of gas unquestionably radiates heat and loses it, and the globe, in consequence of that loss, shrinks to a smaller size. The heat, or what is equivalent to heat, that is left in the globe, is exhibited in a body of reduced dimensions, and in that smaller body the heat shows to such advantage that the ​globe actually exhibits a temperature hotter than before the loss of heat took place.
In the facts just mentioned we have an explanation of the sustained heat of the sun.
Fig. 32.—Solar Eruption, May 3, 1892.
Of course we cannot assume that in our calculations the sun is to be treated as if it were gaseous throughout its entire mass, but it approximates so largely to the gaseous state in the greater part of its bulk that we can feel no hesitation in adopting the belief that the true cause has been found. To justify the adequacy of this method of explaining the facts I may mention the following result of a calculation. If the sun were to lose sufficient heat to enable it to shrink in its diameter by one ten​thousandth part of its present amount, the quantity of heat that would be available in consequence of this contraction would suffice to provide the entire radiation for a period of 2,000 years. Such a diminution of the sun's bulk would be altogether too small to be perceptible by the most refined measurements that we can make in the observatory. Hence we are able to understand how the prodigious radiation of the sun during all the centuries of history can be accounted for without any alteration in the dimensions of the great luminary having yet become appreciable.
But there is a boundary to the prospect of the continuance of the sun's radiation. Of course, as the loss of heat goes on, the gaseous parts will turn into liquids, and as the process is still further protracted, the liquids will transform into solids. Thus we look forward to a time when the radiation of the sun can be no longer carried on in conformity with the laws which dictate the loss of heat from a gaseous body. When this state is reached the sun may, no doubt, be an incandescent solid with a brilliance as great as is compatible with that condition, but the further loss of heat will then involve loss of temperature.
At the present time the body may be so far gaseous that the temperature of the sun remains absolutely constant. It may even be the case that the temperature of the sun, notwithstanding the undoubted loss of heat, is absolutely rising. It is, however, incontrovertible that a certain maximum temperature having been reached (whether we have yet reached it or not we do not know), temperature will then necessarily decline. There is certainly no doubt whatever that the sun, which is now losing heat, even if not actually falling in temperature, must at some ​time begin to lose its temperature. Then, of course, its capacity for radiating heat will begin to abate. The heat received by the earth from the great centre of our system must, of course, decline.
Fig. 33.—Solar Eruption, April 8, 1892.
There seems no escape from the conclusion that the continuous loss of solar heat must still go on, so that the sun will pass through the various stages of brilliant incandescence, of glowing redness, of ​dull redness, until it ultimately becomes a dark and non-luminous star. In this final stage the sun will only pass into that state in which the majority of the bodies in the universe are already found. Every analogy would teach us that the dark and non-luminous bodies in the universe are far more numerous than the brilliant suns. We can never see the dark objects, we can discern their presence only indirectly. All the stars that we can see are merely those bodies which at this epoch of their career happen for the time to be so highly heated as to be luminous.
There is thus a distinct limit to man's existence on the earth, dictated by the ultimate exhaustion of the sun. It is, of course, a question of much interest for us to speculate on the probable duration of the sun's beams in sufficient abundance for the continued maintenance of life. Perhaps the most reliable determinations are those which have been made by Professor Langley. They are based on his own experiments upon the intensity of solar radiation, conducted under circumstances that give them special value. I shall endeavour to give a summary of the interesting results at which he has arrived.
The utmost amount of heat that it would ever have been possible for the sun to contain would, according to this authority, supply its radiation for 40,000,000 years at the present rate. Of course, this does not assert that the sun, as a radiant body, may not be much older than the period named. We have already seen that the rate at which sunbeams are poured forth has gradually increased as the sun rose in temperature. In the early times the quantity of sunbeams dispensed was much less per annum than at present, and it is, therefore, quite possible that the figures may be so enlarged as to meet the requirements ​of any reasonable geological demand with regard to past duration of life on the earth.
It seems that the sun has already dissipated about four-fifths of the energy with which it may have originally been endowed. At all events, it seems that, radiating energy at its present rate, the sun may hold out for 4,000,000 years, or for 5,000,000 years, but not for 20,000,000 years. Here then we discern in the remote future a limit to the duration of life on this globe. We have seen that it does not seem possible for any other source of heat to be available for replenishing the waning stores of the luminary. It may be that the heat was originally imparted to the sun as the result of some great collision between two bodies which were both dark before the collision took place, so that, in fact, the two dark masses coalesced into a vast nebula from which the whole of our system has been evolved. Of course, it is always conceivable that the sun may be re-invigorated by a repetition of a similar startling process. It is, however, hardly necessary to observe that so terrific a convulsion would be fatal to life in the solar system. Neither from the heavens above, nor from the earth beneath, does it seem possible to discover any rescue for the human race from the inevitable end. The race is as mortal as the individual, and, so far as we know, its span cannot under any circumstances be run out beyond a number of millions of years which can certainly be told on the fingers of both hands, and probably on the fingers of one.