M. Arago's Historical Eloge of Joseph Fourier, 219 



Substances differently heated, act on each other, even at great 

 distances, and through a vacuum, for the colder become heated, 

 and the warmer become cool : thus, after a certain interval, 

 they all arrive at the same degree, whatever the original dif- 

 ference of their temperature may have been. 



In the hypothesis which we have mentioned, there is only 

 one mode of explaining this distant action. It consists in sup- 

 posing that it is effected by means of certain effluvia which 

 pass through space from the hot body to the cold body ; and it 

 is asserted that a warm body throws out rays of heat all around 

 itself, as luminous bodies throw out rays of light. 



The effluvia, or radiant emanations, by means of which two 

 bodies at a distance from each other keep up a calorific com- 

 mimication, have been very properly denoted by the term ra- 

 di&nt caloric. 



Radiant caloric had been previously, whatever may have 

 been said to the contrary, the subject of important experiments. 

 before the investigations of Fourier. The celebrated academi- 

 cians del Cimento, found, nearly two centuries ago, that this 

 caloric is reflected like light ; and that, like light, it is concen- 

 trated in the focus of a concave mirror.* By substituting balls 

 of snow for heated bodies, they even proved that frigorific foci 

 may be formed by reflexion. 



Some years afterwards, Mariotte, a member of this academy,, 

 discovered that there are certain kinds of radiant caloric ; and 

 that that which accompanies the solar rays, traverses all tran- 

 sparent media as easily as light ; whilst the caloric which ema- 

 nates from a strongly heated substance before it becomes red 

 hot, as well as the calorific rays which are mixed with the lu- 

 minous rays from a body at a moderate degree of incandescence, 

 are almost completely absorbed in passing through a plate of 

 the most transparent glass ! 



This curious discovery, I may remark, shews, notwithstand- 

 ing the sneers of pretended philosophers, how correct in th^ir 

 ideas the workmen in foundries were, who, from time imme- 

 morial, only looked at the incandescent matter in their fur- 



* This was observed, as well as the apparent reflection of cold, long be- 

 fore, by Baptista Porta. Mag. Nat. p. 6«9. Edit. 1597. F. 



p2 



