ORIGIN AXD AGE OF THE SUITS HEAT. 287 



of our solar system. Beyond our system, the only 

 bodies visible to us are the nebulae and fixed stars, and 

 they are visible because they are luminous. But the 

 fixed stars are beyond doubt suns similar to our own ; 

 and if we assume that the energy in the form of heat 

 and light possessed by our sun has been derived from 

 Motion in Space, we are hardly warranted in denying 

 that the light and heat possessed by the stars were 

 derived from another source. It is true that the motion 

 of the stars in relation to one another, or in relation to 

 our system (and this is the only motion known to us), 

 is but trifling in comparison to what we even witness 

 in our solar system. But this is what we ought, a 

 'priori, to expect; for if their light and heat were 

 derived from Motion in Space, like that of our sun, 

 then, like the sun, they must have lost their motion. 

 In fact, they are suns, and visible because they have 

 lost their motion. Had not the masses of which these 

 suns were composed lost their motion they would have 

 been non-luminous, and of course totally invisible to 

 us. In short, we only see in stellar space those bodies 

 which, by coming into collision, have lost their motion, 

 for it is the lost motion which renders them luminous 

 and visible.* 



* When the foregoing theory of the origin of the sun's heat was 

 advanced, in 1868, I was not aware that a paper on the "Physical 

 Constitution of the Sun and Stars" had been read before the Royal 

 Society by Mr. G. Johnstone Stoney, in which he suggested that the 

 heat possessed by the stars may have been derived from collisions 

 with one another. " If two stars," he says, " should be brought by 

 their proper motion very close, one of three things would happen: — 

 Either they would pass quite clear of one another, in which case 

 they would recede to the same immense distance asunder from which 

 they had come ; or they would become so entangled with one another 

 as to emerge from the frightful conflagration which would ensue, as 

 one star ; or, thirdly, they would brush against one another, but 

 not to the extent of preventing the stars from getting clear again." 



