MARIOTTE’S RESEARCHES ON RADIANT HEAT. 41] 
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 
