CHAPTER XXVI.

THE ASTRONOMICAL SIGNIFICANCE OF HEAT.

Heat and Astronomy-Distribution of Heat-The Presence of Heat in the Earth-Heat in other Celestial Bodies-Varieties of Temperature-The Law of Cooling -The Heat of the Sun-Can its Temperature be Measured?—Radiation connected with the Sun's Bulk-Can the Sun be Exhausting his Resources?—No marked Change has Occurred-Geological Evidence as to the Changes of the Sun's Heat Doubtful-The Cooling of the Sun-The Sun cannot be merely an Incandescent Solid Cooling-Combustion will not Explain the matter-Some Heat is obtained from Meteoric Matter, but this is not Adequate to the Maintenance of the Sun's Heat-The Contraction of a Heated Globe of Gas-An Apparent Paradox-The Doctrine of Energy-The Nebular Theory-Evidence in Support of this Theory-Sidereal Evidence of the Nebular TheoryHerschel's View of Sidereal Aggregation-The Nebulæ do not Exhibit the Changes within the Limits of our Observation.

THAT a portion of a work on astronomy should bear the title placed at the head of this chapter, will perhaps strike some of our readers as unusual, if not actually inappropriate. Is not heat, it may be said, a question merely of experimental physics? and how can it be legitimately introduced into a discussion of the heavenly bodies and their movements? Whatever weight such objections might have once had, need not now be considered. The recent researches on heat have shown not only that heat has important bearings on astronomy, but that it has really been one of the chief agents by which the universe has been moulded into its actual form. At the present time no work on astronomy could be complete without some account of the. remarkable connection between the laws of heat, and the astronomical consequences which follow from obedience to those laws.

In discussing the planetary motions and the laws of Kepler, or in discussing the movements of the moon, the proper motions of the stars, or the revolutions of the binary stars, we proceed on the supposition that the bodies we are dealing with are rigid

particles, and the question as to whether these particles are hot or cold does not seem to have any especial bearing. No doubt the ordinary periodic phenomena of our system, such as the revolution of the planets in conformity with Kepler's laws, will be observed for countless ages, whether the planets be hot or cold, or whatever may be the heat of the sun. It must, however, be admitted that the laws of heat introduce certain modifications into the statement of these laws. The effects of heat may not be immediately perceptible, but they exist, they are constantly acting, and in the progress of time they are adequate to effecting the mightiest changes throughout the universe.

Let us briefly recapitulate the circumstances of our system which give to heat its potency. Look first at our earth, which at present seems-on its surface, at all events-to be a body devoid of heat; but a closer examination will at once dispel this idea. Have we not the phenomena of volcanoes, of geysers, and of hot springs, which show that in the interior of the earth heat must exist in

far greater intensity than we find on the surface? These phenomena are found in widely different regions of the earth. Their origin is, no doubt, involved in a good deal of obscurity, but yet no one can deny that they indicate vast reservoirs of heat. It would indeed seem that heat is to be found everywhere in the deep inner regions of the earth. If we take a thermometer down a deep mine, we find it records a temperature higher than at the surface. The deeper we descend the higher is the temperature; and if the same rate of progress should be maintained through those depths of the earth which we are not able to penetrate, it can be demonstrated that at twenty or thirty miles below the surface the temperature must be as great as that of red-hot iron.

We find in the other celestial bodies abundant evidence of the present or the past existence of heat. Our moon, as we have already mentioned, affords a very striking instance of a body which must once have been very highly heated. The extraordinary volcanoes on its surface render this beyond any doubt. It is equally certain that those volcanoes have been silent for ages, so that, whatever may be the interior condition of the moon, the surface

has now cooled down. Extending our view further, we see in the great planets Jupiter and Saturn evidence that they are still endowed with a temperature far in excess of that which the earth has retained; while when we look at our sun, we see a body in a state of brilliant incandescence, and glowing with a fervour to which we cannot approximate in our mightiest furnaces. The various fixed stars are bodies which glow with heat, like our sun; while we have in the nebulæ objects whose existence is hardly intelligible to us unless we admit that they are possessed of a vast store of heat.

From this rapid survey of the different bodies in our universe, one conclusion is obvious. We may have great doubts as to the actual temperature of any individual body of the system; but it cannot be doubted that there is a wide range of temperature among the different bodies. Some are hotter than others. The stars and suns are perhaps the hottest of all, but it is not improbable that they may be immeasurably outnumbered by the cold and dark bodies of the universe, which are to us invisible, and only manifest their existence in an indirect and casual manner.

The law of cooling tells us that every body radiates heat, and that the quantity of heat which it radiates increases when the temperature of the body increases relatively to the surrounding medium. This law appears to be universal. It is obeyed on the earth, and it would seem that it must be equally obeyed by every other body in space. We thus see that each of the planets and each of the stars is continuously pouring forth in all directions a never-ceasing stream of heat.

This radiation of heat is productive of very momentous consequences. Let us study them, for instance, in the case of the sun. Our great luminary pours forth a mighty flood of radiant heat in all directions. A minute fraction of that heat is intercepted by our earth, and is directly or indirectly the source of all life, and of nearly all movement, on our earth. To pour forth heat as the sun does, it is necessary that his temperature be enormously high. And there are some facts which permit us to form an estimate of what that temperature must actually be.

The tempera

Every one is acquainted with the use of a burning-glass, by which we can condense the sun's rays to a focus, and produce incandescence or set objects on fire. Large burning-glasses have been constructed, in the focus of which an extraordinary temperature has been obtained. It can, however, be proved that the temperature at the focus cannot be greater, cannot be even equal, to the temperature at the source of heat itself. The effect of a burning-glass is merely equivalent to making a closer approach towards the sun. The rule is indeed a simple one. ture at the focus of the burning-glass is the same as that of a point placed at such a distance from the sun that the solar disc would seem just as large as the lens itself viewed from its own focus. The greatest burning-glass which has ever been constructed virtually transports an object at its focus to within 250,000 miles of the sun's surface: in other words, to a distance of about 1-400th part of its present amount. In this focus it was found that the most refractory substances, agate, cornelian, platinum, fire-clay, the diamond itself, were melted or even dissipated into vapour. There can be no doubt that if the sun were to come as near to us as the moon, the solid earth itself would melt like wax.

It is difficult to form any numerical statement of the actual temperature of the sun. The intensity of that temperature vastly transcends the greatest artificial heat, and any attempt to clothe such estimates in figures is necessarily very precarious. But assuming the greatest artificial temperature to be about 4,000° Fahr., we shall probably be well within the truth if we state the effective temperature of the sun to be about 18,000° Fahr. This is, indeed, vastly below many of the estimates which have been made. Secchi, for instance, has estimated the sun's temperature to be nearly one thousand times that here given.

The copious outflow of heat from the sun corresponds with its enormous temperature. We can express the amount of heat in various ways, but it must be remembered that considerable uncertainty still attaches to such measurements. The old method of measuring heat by the quantity of ice melted may be used as an illustration. It is computed that a shell of ice 43 feet thick

surrounding the whole sun would in one minute be melted by the sun's heat underneath. A somewhat more elegant illustration was also given by Sir John Herschel, who showed that if a cylindrical glacier forty-five miles in diameter were to be continually flowing into the sun with the velocity of light, the end of that glacier would be melted as quickly as it advanced. From each square foot on the surface of the sun emerges a quantity of heat as great as could be produced by the daily combustion of sixteen tons of coal. This is, indeed, an amount of heat which, properly transformed into work, would keep an engine of many hundreds of horse-power running from one year's end to the other. The heat radiated from a few acres on the sun would be adequate to drive all the steamengines in the world. When we reflect on the vast intensity of the radiation from each square foot of the sun's surface, and when we combine with this the stupendous dimensions of the sun, imagination fails to realise how vast must be the actual expenditure of heat.

In one way the enormous intensity of the radiation from each unit of sun surface is a consequence of its bulk. Imagine for a moment two suns, one of which had a diameter double the other. If these two suns were of analogous constitution, their stores of heat may be taken as proportional to their volumes. The larger sun would thus have a store of heat eight times as great as the smaller one. But the ratio of the surfaces of the two suns is only four to one. Hence it follows that by the time both suns cooled down, twice as much heat per unit of area must have passed through the surface of the large sun as through the surface of the small one. To emphasize the contrast still more, suppose our present sun compared with a fictitious sun not larger than our earth. If the two suns started under equal circumstances, then before they had both cooled down to the same temperature, nearly one hundred times as much heat must be transmitted through each square foot of the sun's surface as through each square foot of the earth's surface.

In presence of the beneficent, if prodigal, expenditure of the sun's heat, we are tempted to ask a question which has the most vital interest for the earth and its inhabitants. We live from