284 DISCUSSIONS IN COSMOLOGY. 



mass have become incandescent? From what source 

 could the heat have been obtained ? The dynamical 

 theory of heat affords, as was shown several years 

 ago * an easy answer to this question. The answer is 

 that the energy in the form of heat possessed by the 

 mass may have been derived from Motion in Space. 

 Two bodies, each one-half the mass of the sun, moving 

 directly towards each other with a velocity of 476 

 miles per second, would by their concussion generate 

 in a single moment 50 million years' heat. For two 

 bodies of that mass, moving with a velocity of 476 

 miles per second, would possess 4149 X 10 38 foot-pounds 

 of kinetic energy, and this converted into heat by the 

 stoppage of their motion would give out an amount of 

 heat which would cover the present rate of the sun's 

 radiation for a period of 50 million years.-f" 



Why may not the sun have been composed of two 

 such bodies ? And why may not the original store of 

 heat possessed by him have all been derived from the 

 concussion of these two bodies ? Two such bodies 

 coming into collision with that velocity would be dissi- 

 pated into vapour and converted into a nebulous mass 

 by such an inconceivable amount of heat as would 

 thus be generated ; and when condensation on cooling 

 took place, a spherical mass like that of the sun 

 would result. It is perfectly true that two such bodies 

 could never attain the required amount of velocity by 

 their mutual gravitation towards each other. But 

 there is no necessity whatever for supposing that 

 their velocities were derived from their mutual attrac- 

 tion alone : they might have been approaching each 



* "Phil. Mag." for May, 1868. 



f It does not necessarily follow, of course, that the two bodies 

 coming into collision should possess equal mass or velocity in order 

 to have their motion of translation converted into heat. 



