410 JOSEPH FOURIER. 



which is eminently worthy of being studied between the 

 ball of iron at the ordinary temperature which may be 

 handled at pleasure, and the ball of iron of the same 

 dimensions which the flame of a furnace has very much 

 heated, and which we cannot touch without burning our- 

 selves. This distinction, according to the majority of 

 physical inquirers, arises from a certain quantity of an 

 elastic imponderable fluid, or at least a fluid which has 

 not been weighed, with which the second ball has com- 

 bined during the process of heating. The fluid which, 

 upon combining with cold bodies renders them hot, has 

 been designated by the name of heat or caloric. 



Bodies unequally heated act upon each other even at 

 great distances, even through empty space, for the colder 

 becomes more hot, and the hotter becomes more cold ; 

 for after a certain time they indicate the same degree of 

 the thermometer, whatever may have been the difference 

 of their original temperatures. According to the hypoth- 

 eses above explained, there is but one way of conceiving 

 this action at a distance ; this is to suppose that it oper- 

 ates by the aid of certain effluvia which traverse space 

 by passing from the hot body to the cold body ; that is, 

 to admit that a hot body emits in every direction rays of 

 heat, as luminous bodies emit rays of light. 



The effluvia, the radiating emanations by the aid of 

 which two distant bodies form a calorific communication 

 with each other, have been very appropriately designated 

 by the name of radiating caloric. 



Whatever may be said to the contrary, radiating heat 

 had already been the object of important experiments 

 before Fourier undertook his labours. The celebrated 

 academicians of the Cimento found, nearly two centuries 

 ago, that this heat is reflected like light ; that, as in the 



