CONDUCTION AND RADIATION. 543 



laws. It is an obvious and general fact, that bodies 

 which are included in the same space tend to ac- 

 quire the same temperature. And this identity of 

 temperature of neighbouring 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 surface 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 supposition, a ray emitted obliquely from an 

 internal particle, will be less intense than one sent 

 forth pependicular to the surface, because the for- 

 mer 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 intensity is as the sine of the angle made 

 by the ray with the surface. 



But this law is, as I have said, likewise neces- 

 sary, in order that neighbouring bodies may tend 

 to assume the same temperature : for 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. 

 " Mem. Inxl. t. v. 1821, p. 204. M An. Chim. iv. 1817, p. 129. 



