CH. XXXV. CONSERVATION OF ENERGY. 361 



Thomson, has come to a conclusion which we must try to 

 understand, however imperfectly. 



We have already seen when speaking of Hirn's engine 

 that it is impossible to convert all the heat passing through 

 the engine into work. Now, that portion which escapes in 

 the form of heat, although it represents quite as much 

 mechanical energy as it did before, is no longer useful in the 

 same way, because it has become spread out or diffused ; and 

 this happens whenever energy changes its form, whether 

 from mechanical energy into electrical energy, from heat 

 into work, from work into heat, or in any other way. In- 

 variably a certain amount of energy is dissipated, or spread 

 out in the form of diffused heat. 



The same amount of energy remains, it is true, but it is 

 so uniformly distributed as to be no longer available for 

 work ; for heat can only be turned into work when it passes 

 from a hotter to a colder body ; therefore heat spread equally 

 over all bodies in nature is lost as a working power, even 

 though it represents the same amount of energy, which could 

 be made to do work if it could be collected again into a 

 condensed form. 



This fact of the dissipation of energy was first announced 

 by Sir William Thomson in 1852, and he pointed out that 

 it tends to let down, as it were, the working power of nature. 

 We keep up our store by deriving fresh heat from the sun, 

 and storing it up in the vegetable world, in wood, and coal, 

 and also in the falling power of water which has been drawn 

 up to the clouds by the sun's heat. But the sun's heat itself, 

 and every operation going on in the universe, is caused by the 

 transformation of energy, therefore dissipation must also go on 

 with it ; and, unless there is some compensating power, the 

 whole universe must be drifting very slowly into a state of 

 rest 



