380 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



of the universe, through all distances of place or time. 

 The universe appears, according to this law, to be en- 

 dowed with a store of energy which, through all the 

 varied changes in natural processes, can neither be 

 increased nor diminished, which is maintained therein 

 in ever-varying phases, but, like matter itself, is from 

 eternity to eternity of unchanging magnitude ; acting 

 in space* but not divisible, as matter is, with it. Every 

 change in the world simply consists in a variation in the 

 mode of appearance of this store of energy. Here we 

 find one portion of it as the vis viva of moving bodies, 

 there as regular oscillation in light and sound ; or, again, 

 as heat, that is to say, the irregular motion of invisible 

 particles ; at another point the energy appears in the 

 form of. the weight of two masses gravitating towards 

 each other, then as internal tension and pressure of 

 elastic bodies, or as chemical attraction, electrical ten- 

 sion, or magnetic distribution. If it disappear in one 

 form, it reappears as surely in another ; and whenever 

 it presents itself in a new phase we are certain that 

 it does so at the expense of one of its other forms. 



Carnot's law of the mechanical theory of heat, as 

 modified by Clausius, has, in fact, made it clear that 

 this change moves in the main continuously onward in a 

 definite direction, so that a constantly increasing amount 

 of the great store of energy in the universe is being 

 transformed into heat. 



We can, therefore, see with the mind's eye the original 

 condition of things in which the matter composing the 

 celestial bodies was still cold, and probably distributed 

 as chaotic vapour or dust through space ; we see that 

 it must have developed heat when it collected together 

 under the influence of gravity. Even at the present 

 time spectrum analysis (a method the theoretical prin- 

 ciples of which owe their origin to the mechanical theory 



