292 THE LESSON OF EVOLUTION 



place, to divide stores of mechanical energy into two 

 classes static and kinetic. A quantity of weights at 

 a height, ready to descend and do work when wanted, 

 an electrified body, a quantity of fuel, contain 

 stores of mechanical energy of the static kind. 

 Masses of matter in motion, a volume of space 

 through which undulations of light or radiant heat 

 are passing, a body having thermal motions among 

 its particles (that is, not infinitely cold), contain 

 stores of energy of the kinetic kind. 



The following propositions are laid down regarding 

 the dissipation of mechanical energy from a given 

 store, and the restoration of it to its primitive con- 

 dition. They are necessary consequences of the 

 axiom, "It is impossible by means of inanimate 

 " material agency, to derive mechanical effect from 

 " any portion of matter by cooling it below the 

 "temperature of the coldest of the surrounding 

 " objects." (Dynamical Theory of Heat [Art. 

 XL VIII, Vol. i., "Collected Math, and Phys. 

 Papers"] 12). 



I. When heat is created by a reversible process (so 

 that the mechanical energy thus spent may be 

 restored to its primitive condition), there is also a 

 transference from a cold body to a hot body of a 

 quantity of heat bearing, to the quantity created, a 

 definite proportion depending on the temperatures of 

 the two bodies. 



II. When heat is created by any unreversible pro- 

 cess (such as friction), there is a dissipation of 

 mechanical energy, and a full restoration of it to its 

 primitive condition is impossible. 



