542 HISTORY OF THERMOTICS. 



surface of a hot body in all directions ; but by no 

 means in all directions with equal intensity. The 

 intensity of the heating ray is as the sine of the 

 angle which it makes with the surface. 



The last law is entirely, the two former in a 

 great measure, due to the researches of Leslie, 

 whose Experimental Inquiry into the Nature and 

 Propagation of Heat, published in 1804, contains 

 a great number of curious and striking results and 

 speculations. The laws now just stated bear, in a 

 very important manner, upon the 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 Hadiant Neat. 



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 shown that the supposition 

 to which we are led by the most simple and general 

 of the facts, will explain, moreover, the less obvious 



