I2 4 POPULAR LECTURES AND ADDRESSES. 



coalition of its parts amounts to x 6,370,000 or 

 4,250,000 metre-tons per ton of its whole mass : 

 the metre-ton (an ordinary gravitation unit of 

 work) being the amount of work required to over- 

 come, through a space of one metre, a force equal 

 to the weight of a ton at the earth's surface ; the 

 difference of the force of gravity at different parts 

 of the earth's surface neglected. But unless, which 

 is very improbable, the conglomeration took place 

 quite suddenly by the simultaneous collision of 

 materials falling in from all sides, a large part of 

 this energy must have been dissipated away by 

 radiation of heat consequent on partial collisions. 

 We must therefore look on the definite estimate 

 4,250,000 metre-tons per ton of the earth's mass, 

 which expresses somewhat accurately the whole 

 potential energy exhausted during the conglomera- 

 tion, as being considerably above the greatest 

 amount of plutonic energy due to gravitation, 

 that can ever have existed in the earth at any 

 one time. 



38. To estimate the potential energy of chemical 

 affinity already exhausted, or yet to be exhausted, 



