JOSEPH FOURIER. iO < 



separately coui^idered. I sliall adopt the same division, commencing, 

 however, with radiant beat the historical analysis wliicli I am about 

 to submit to you. 



Nobody doubts that there is a physical distiuctiou which is eminently 

 worthy of being studied between the ball of iron at the ordinary temper- 

 ature 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 ourselves. 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 beeu weighed, with which the second ball has combined during the 

 process of heating. The fluid which upon combining with cold bodies 

 renders them hot, has beeu designated by the name of heat or caloric. 



Bodies unequally' heated act upon each other even at great (listanccs, 

 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 hypotheses above explained, 

 there is but one way of conceiving this action at a distance : this is to 

 suppose that it operates by the aid of certain efliuvia which traverse 

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

 that a hot bodj^ emits in every direction rays of heat, as luminous bodies 

 emit rays of light. 



The efliuvia, 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 

 labors. The celebrated Academicians of the Cimento found, nearly two 

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

 of light, a concave mirror concentrates it at the focus. Upon substi- 

 tuting balls of snow for heated bodies, they even went so far as to prove 

 that frigorific foci may be formed b^^ way of reflection. Some years 

 afterward Mariotte, a member of this Academy, discovered that there 

 exist diftereut kinds of radiating heat ; that th(^, heat with which rays 

 of light are accompanied traverses all transparent media as easily as 

 light does ; while, again, the caloric which emanates from a strongly 

 heated, but opaque substance, as well as the rays of heat which are found 

 mingled with the luminous rays of a body moderately incandescent, are 

 almost entirely arrested in their passage through the most transparent 

 plat<3 of glass! 



This striking discovery, let us remark in passing, will show, notwith- 

 standing the ridicule of pretended savants, how happily ius]Mred were 

 the workmen in founderies, who looked at the incandescent matter of 

 their furnaces only through a plate of ordinary glass, thinking by the 



