M. Arago's Historical Eloge of Joseph Fourier. 221 



common phenomena of temperature, in phenomena which, at 

 first sight, seem to be perfectly unconnected with it. 



Such is the privilege of genius ; it perceives, it seizes on re- 

 lations where ordinary eyes only see isolated facts. 



Nobody doubts, and, besides, experience has shewn, that, in 

 all the points of any space contained within certain boundaries, 

 and su])ported at a constant temperature, we cannot find a tem- 

 perature both constant and precisely the same as that of the 

 envelope. Now Fourier has established, that, if the emitted 

 calorific rays had an equal intensity in all directions, that if this 

 same intensity did not vary proportionally to the sine of the 

 angle of emission, the temperature of a body in the interior 

 would depend on the situation which it occupied in it : that 

 the temperature of boiling water or of melted iron, for in- 

 stance, would exist at cei'taln points of a hollow envelope of ice!! 

 Within the vast range of the physical sciences, we could not 

 find a more striking application of the celebrated method of 

 reductio ad absurdum, which the old mathematicians employed 

 to demonstrate the abstract truths of geometry. 



I shall not pass from this first portion of the works of Fou- 

 rier, without adding, that he did not rest satisfied with point- 

 ing out so happily the remarkable law which connects the 

 comparative intensities of the calorific rays thrown out at all 

 angles from heated bodies ; but he also examined into the 

 physical cause of this law, and discovered it in a circumstance 

 which his predecessors had entirely overlooked. Let us sup- 

 pose, said he, that bodies emit heat not only from their super- 

 ficial particles, but also from those in the interior. Let us ad- 

 mit, moreover, that the heat of these latter cannot arrive at the 

 surface, by passing through a certain quantity of matter, with- 

 out experiencing some absorption. Fourier reduced to calcu- 

 lation these two hypotheses, and deduced mathematically from 

 them the experimental law of the sine. After standing so 

 complete a test, the two hypotheses were completely confirmed ; 

 they became laws of nature, and represented, in caloric, hidden 

 properties which could only be mentally appreciated. 



In the second question treated of by Fourier, heat is pre- 

 sented under a new form. There is more difficulty in follow- 



