138 POPULAR SCIENCE MONTHLY. 



the fact that the Sun and stars have, for untold millions of years, been 

 radiating heat into space. If we refrain from considering the basis on 

 which this conclusion rests, it is not so much because we consider it un- 

 questionable, as because the discussion would be too long and complex 

 for the present work. 



One of the great problems of modern science has been to account 

 for the source of this heat. Before the theory of energy was developed 

 this problem offered no difficulty. In the time of Newton, Kant and 

 even of La Place and Herschel, no reason was known why the stars 

 should not shine forever without change. Now we know that when a 

 body radiates heat, that heat is really an entity termed energy, of which 

 the supply is necessarily limited. Kelvin compared the case of a star 

 radiating heat with that of a ship of war belching forth shells from her 

 batteries. We know that if the firing is kept up, the supply of am- 

 munition must at some time be exhausted. Have we any means of deter- 

 mining how long the store of energy in Sun or star will suffice for its 

 radiation? 



We know that the substances which mainly compose the Sun and 

 stars are similar to those which compose our earth. We know the 

 capacity for heat of these substances, and we also have determined how 

 much the Sun radiates annually. From these data, it is found by a sim- 

 ple calculation that the temperature of the Sun would be lowered annu- 

 ally by more than two degrees Fahrenheit, if its capacity for heat were 

 the same as that of water. If this capacity were only that of the sub- 

 stances which compose the great body of the earth, the lowering of tem- 

 perature would be from 5° to 10° annually. Evidently, therefore, the 

 actual heat of the Sun would only suffice for a few thousand years' 

 radiation, if not in some way replenished. 



When the difficulty was first attacked, it was supposed that the sup- 

 ply might be kept up by meteors falling into the Sun. We know that in 

 the region round the Sun, and, in fact, in the whole Solar System, are 

 countless minute meteors some of which may from time to time strike 

 the Sun. The amount of heat that would be produced by the loss of 

 energy suffered by a meteor moving many hundred miles a second 

 would be enormously greater than that which would be produced by 

 combustion. But critical examination shows that this theory cannot 

 have any possible basis. Apart from the fact that it could at best be 

 only a temporary device there seems to be no possibility that meteors 

 sufficient in mass can move round the Sun or fall into it. Shooting 

 stars show that our earth encounters millions of little meteors every 

 day; but the heat produced is absolutely insignificant. 



It was then shown by Kelvin and Helmholtz that the Sun might 

 radiate the present amount of heat for several millions of years, simply 

 from the fund of energy collected by the contraction of its volume 



