uS POPULAR LECTURES AND ADDRESSES. 



which constitute plutonic action, consists certainly 

 at the present time in a great measure, if not 

 altogether, of terrestrial heat. This indeed is the 

 only description of energy proved to exist in any 

 considerable quantity within the earth ; but it is 

 possible that there may be great masses of uncom- 

 bined chemical elements, and that the potential 

 energy of their mutual affinities may constitute a 

 considerable portion of the plutonic energy in store, 

 whether for the generation of future underground 

 heat, or for immediate application to some of the 

 more violent manifestations of plutonic activity. 

 Now, there are two ways of estimating the possible 

 total amount of plutonic energy ; one by taking 

 the earth as it is, and not reasoning from ante- 

 cedent conditions, but simply estimating from 

 known properties of matter ; how much heat it is 

 conceivable may exist in it in its present condition ; 

 the other by tracing the history of the earth 

 backwards. 



34. From experiments such as have not yet been 

 made, but could be made with very great ease, on 

 the total heats of fusion of ordinary rocks and 



