THERMOGALVANIC CURRENTS. 219 



increased. The several points C, C, C, are usually 



Fig. 74. 



brought very near together, in order to receive the 

 same degree of heat. 



This is the most delicate method known for mea- 

 suring the intensity of radiant heat. It is thus that 

 the diathermancy of different kinds of glass and of 

 different crystals, &c., have been compared with great 

 accuracy, and that the radiation of heat from the 

 principal fixed stars has been made sensible. 



Regarding the earth as a heterogeneous compound 

 of different substances which may possess in some 

 degree the properties of different metals, and conceiv- 

 ing (as is the opinion of many physicists) that there 

 is in the interior a great store of caloric, which may 

 heat the points of contact, some of them steadily and 

 some by occasional bursts of flame, it seems within the 



