RADIATION. 177 



bulb. Heat is radiated from every part of the room, from 

 the freezing mixture as well as from the walls, but there is 

 less heat radiated from the freezing mixture than from 

 other parts, whereas the bulb of the thermometer will be 

 radiating heat not only to the walls of the room, but also 

 to the mirror and thence to the freezing mixture. There is 

 a continual exchange of heat going on between the thermo 

 meter and surrounding bodies, if they are all at the same 

 temperature the thermometer does not change, because it 



FIG. 1. 



is radiating as much heat as it is absorbing; but if we 

 place a freezing mixture in that focus, the thermometer 

 will radiate to that freezing mixture a certain amount of 

 heat greater than the amount of heat it gets from it. 



After this was stated by Pictet in Geneva, there being a 

 large band of philosophers bound together at Geneva, they 

 continued to take up this, and to examine the laws of 

 radiant heat with some care. The most important results 

 that were obtained were those obtained by M. Prevost. 

 He published a theory of the law of exchanges which has 



N 



