L52 HISTORY OF THERMOTICS. 



-tat eel bear, in a very important manner, upon tlie formation of the 

 theory ; and we must now proceed to consider what appears to have 

 been done in this respect ; taking into account, it must still be borne 

 in mind, only the phenomena of conduction and radiation. 



. Sect. 7. Fourier's Theory of Radiant Heat. 



THE above laws of phenomena being established, it was natural that 

 philosophers should seek to acquire some conception of the physical 

 action by which they might account, both for these laws, and for the 

 general fundamental facts of Thermotics ; as, for instance, the fact that 

 all bodies placed in an inclosed space assume, in time, the temperature 

 of the inclosure. Fourier's explanation of this class of phenomena 

 must be considered as happy and successful ; for he has show r n that 

 the supposition to which we are led by the most simple and general 

 of the facts, will explain, moreover, the less obvious laws. It is an 

 obvious and general fact, that bodies which are included in the space 

 tend to acquire the same temperature. And this identity of tempera- 

 ture of neighboring bodies requires an hypothesis, which, it is found, 

 also accounts for Leslie's law of the sine, in radiation. 



This hypothesis is, that the radiation takes place, not from the snr- 

 ;ai,e alone of the hot body, but from all particles situated within a 

 certain small depth of the surface. It is easy to see 22 that, on this sup- 

 position, a ray emitted obliquely from an internal particle, will be less 

 intense than one sent forth perpendicular to the surface, because the 

 former will be intercepted in a greater degree, having a greater length 

 of path within the body; and Fourier shows, that whatever be the law 

 of this intercepting power, the result will be, that the radiative inten 

 f-ity is as the sine of the angle made by the ray with the surface. 



But this law is, as I have said, likewise necessary, in order that 

 neighboring bodies may tend to assume the same temperature : fur 

 instance, in order that a small particle placed within a spherical 

 shell, should finally assume the temperature of the shell. If the law 

 of the sines did not obtain, the final temperature of such a particle 

 would depend upon its place in the inclosure; 23 and within a shell of 

 ice we should have, at certain points, the temperature of boiling water 

 and of melting iron. 



This proposition may at first appear strange and unlikely ; but it may 



"Mem. lust. t. v. 1821, p. 201. - s .-l. Cfiim. iv. 1817, p. 129. 



