MARIOTTE'S RESEARCHES ON RADIANT HEAT. 411 



case of light, a concave mirror concentrates it at the 

 focus. Upon substituting balls of snow for heated bodies, 

 they even went so far as to prove that frigorific foci may 

 be formed by way of reflection. Some years afterwards 

 Mariotte, a member of this Academy, discovered that 

 there exist different kinds of radiating heat ; that the 

 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, while the rays of heat, which are 

 found mingled with the luminous rays of a body moder- 

 ately incandescent, are almost entirely arrested in their 

 passage through the most transparent plate of glass ! 



This striking discovery, let us remark in passing, will 

 show, notwithstanding the ridicule of pretended savans, 

 how happily inspired were the workmen in founderies, 

 who looked at the incandescent matter of their furnaces, 

 only through a plate of ordinary glass, thinking by the 

 aid of this artifice to arrest the heat which would have 

 burned their eyes. 



In the experimental sciences, the epochs of the most 

 brilliant progress are almost always separated by long in- 

 tervals of almost absolute repose. Thus, after Mariotte, 

 there elapsed more than a century without history having 

 to record any new property of radiating heat. Then, in 

 close succession, we find in the solar light obscure calo- 

 rific rays, the existence of which could admit of being 

 established only with the thermometer, and which may 

 be completely separated from luminous rays by the aid of 

 the prism ; we discover, by the aid of terrestrial bodies, 

 that the emission of caloric rays, and consequently the 

 cooling of those bodies, is considerably retarded by the 

 polish of the surfaces ; that the colour, the nature, and 



