A PREAMBLE ON ENERGY 67 



Such examples can be multiplied. They show that 



(/) Energy which can be made to transform passes wholly 

 into heat ; or 



{ii) if it can be made to transform into some other energy- 

 form some of it does not do so but transforms into heat ; 



{iii) and either all of the heat so transformed, or some of it, 



cannot be made to undergo any further transformation : that 



is, it becomes unavailable for further physical changes, or 



transformations. 



and, therefore, the equivalence M = Q h true in one sense 



M = Q^ but it is not true in the other sense O = M. This 



statement holds good for human experience, " so far as it goes." 

 It means that in all known physical processes some of the involved 

 available energy that transforms, or " keeps things going," is 

 actually expended and, so far as our power goes, is annihilated 

 or disappears. Or physical causality is continually being lost in 

 the universe, as we know it. 



22^. Entropy. The precise statement of the above matters 

 is contained in the mathematical function called entropy. We 

 consider the flow of heat. Let a system of things have a certain 

 quantity of heat energy = Q. This heat will only flow, of itself, 

 into another system of things if this latter system is at a lower 

 temperature than the former one. (Obviously, for instance, the 

 heat of a coal fire flows into, or warms up, a cold room.) 



We say that the entropy of the first system is — ~ (where T^ 



is the absolute temperature, that is, ordinary Centigrade tempera- 

 tures -f 273°). Let O units of heat flow from this system, then, 



the latter loses — entropy. But the second system, which is at 



a lower temperature than the first one, receives the heat that 

 flows (if it were not at a lower temperature heat would not flow 

 of itself). The temperature of this latter system is T^2- There- 

 fore it gains — entropy : and the O's being the same, while T 



'-po 



o 



2 



2 



is less than T^, the entropy, — , is greater than — ~. And, there- 



J. 2 ^ 



fore, entropy increases when heat flows, or is transformed in any 

 actual way known to us. 



