422 POPULAR LECTURES AND ADDRESSES. 



fragments to prove that the heat developed by 

 the friction is not produced from the solids but is 

 called into existence between them. An unfortunate 

 use of the word "capacity for heat," which has 

 been the occasion of much confusion ever since 

 the discovery of latent heat, and has frequently 

 obstructed the natural course of reasoning on 

 thermal and thermo-dynamic phenomena, appears 

 to have led both Rumforcl and Davy to give 

 reasoning which no one could for a moment feel to 

 be conclusive, and to have prevented each from 

 giving a demonstration which would have estab- 

 lished once and for ever the immateriality of heat. 

 Another case of apparent loss of work, well 

 known to an audience in the Royal Institution 

 that in which a mass of copper is compelled to 

 move in the neighbourhood of a magnet was 

 adduced ; and an experiment was made to demon- 

 strate that in it also heat appears as an effect of 

 the work which has been spent. A copper ball, 

 about an inch in diameter, was forced to rotate 

 rapidly between the poles of a powerful electro- 

 magnet. After about a minute it was found by 



