CONDUCTION AND RADIATION. -V2f> 



body cools down. The first step in our systematic 

 knowledge of this subject was made in the Prin- 

 cipia. " It was in the destiny of that great work," 

 says Fourier, "to exhibit, or at least, to indicate, 

 the causes of the principal phenomena of the uni- 

 verse." Newton assumed, as we have already said, 

 that the rate at which a body cools, that is, parts 

 with its heat to surrounding bodies, is proportional 

 to its heat ; and on this assumption he rested the 

 verification of his scale of temperatures. It is an 

 easy deduction from this law, that if times of cool- 

 ing be taken in arithmetical progression, the heat 

 will decrease in geometrical progression. Kraft, 

 and after him Richman, tried to verify this law 

 by direct experiments on the cooling of vessels of 

 warm water ; and from these experiments, which 

 have since been repeated by others, it appears that 

 for differences of temperature which do not exceed 

 50 degrees (boiling water being 100), this geome- 

 trical progression represents, with tolerable (but 

 not with complete) accuracy, the process of cooling. 

 This principle of radiation, like that of conduc- 

 tion, required to be followed out by mathematical 

 reasoning. But it required also to be corrected in 

 the first place, for it was easily seen that the rate of 

 cooling depended, not on the absolute temperature 

 of the body, but on the excess of its temperature 

 above the surrounding objects to which it com- 

 municated its heat in cooling. And philosophers 

 were naturally led to endeavour to explain or illus- 



