HISTORY OF THEKMOTICS. 



Sect. 2. Introduction of the Doctrine of Radiation. 



A HOT body, as a mass of incandescent iron, emits heat, as we per> 

 ccive by our senses when we approach it ; and by this emission of 

 heat the hot body cools down. The first step in our systematic know- 

 ledge of the subject was made in the Principia. "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 universe." 

 Xewton assumed, as we have already said, that the rate at which a 

 body cools, that is, parts with its heat to surrounding bodies, is pro- 

 portional 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 cooling be taken in arithmetical progression, the heat 

 will decrease in geometrical progression. Kraft, and after him Rich- 

 man, 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 geo- 

 metrical progression represents, with tolerable (but not with complete) 

 accuracy, the process of cooling. 



This principle of radiation, like that of conduction, 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 cool- 

 ing depended, not on the absolute temperature of the body, but on 

 the excess of its temperature above the surrounding objects to which 

 it communicated its heat in cooling. And philosophers were naturally 

 led to endeavor to explain or illustrate this process by some physical 

 notions. Lambert in 1755 published 5 an Essay on the Force of Heat, 

 in which he assimilates the communication of heat to the flow of a 

 fluid out of one vessel into another by an excess of pressure ; and 

 mathematically deduces the laws of the process on this ground. But 

 some additional facts suggested a different view of the subject. It was 

 found that heat is propagated by radiation according to straight lines, 

 like light ; and that it is, as light is, capable of being reflected by 

 mirrors, and thus brought to a focus of intenser action. In this man- 

 ner the radiative effect of a body could be more precisely traced. A 

 fact, however, came under notice, which, at first sight, appeared tc 



Aft. Helvet. torn. ii. p. 172. 



