OF RADIANT HEAT THROUGH DIFFERENT BODIES. 68 
diaphanous substances intercept them in very different quantities, and 
the portions transmitted are considerably diminished by increasing the 
thickness of the flakes. Thus, the rays emerging from the black or the 
green glasses are in respect to their properties of transmission as it were 
antagonist to the preceding, and analogous to those of the direct heat 
of the flame though still more decidedly marked, for they are almost 
completely absorbed by bodies possessing the greatest transparency. 
I have availed myself of these last facts for the purpose of proving by 
a very simple process that solar light contains some calorific rays analo- 
gous to those which compose the radiant heat of terrestrial sources. 
With this view I introduced a solar ray into a dark room through an 
aperture having a screen of green glass as a stopper. To the light 
transmitted I exposed one of the blackened balls of a very delicate dif- 
ferential thermometer. The liquid column descended several degrees. 
I now placed quite close to the mouth of the aperture a thin plate 
of colourless glass; the liquid came back a little, but the retrograde 
movement became more decided when I interposed instead of the thin 
glass a plate of greater thickness. I took away the white glass and put 
in its place a plate of rock salt: the column was forcibly driven back, 
but reascended very nearly to its original position when I substituted 
for the salt a plate of very limpid alum. It is clear therefore that 
amongst the calorific rays of the sun there are some which have a re- 
semblance to terrestrial heat. On the other hand we have seen that the 
rays from terrestrial flame which traverse a flake of alum suffer, like 
solar heat, only a very slight diminution in passing through glass and 
other diaphanous substances. Whence we infer that amongst the ca- 
lorific rays from flame some are found similar to the heat of the sun. 
The differences observed between solar and terrestrial heat, as to their 
properties of transmission, are therefore to be attributed merely to the 
mixture, in different proportions, of several species of rays. 
But, to return to the heat emerging from the screens exposed to the 
radiation of the lamp. We have said that the red, orange, yellow, blue, in- 
digo, and violet matters which enter into the composition of the coloured 
glasses, act upon radiant heat as the black substances introduced into a 
coloured medium act relatively to light ; that is, they diminish the quan- 
tity of heat transmitted by the glass without altering its diathermancy 
[diathermansie]. This proposition being admitted, it will necessarily 
follow, when rays of different species, such as issue from the five screens 
contained in the table, fall on a series of coloured glasses, that the ca- 
lorific transparencies of these plates will be increased or diminished in 
proportion to the variation produced in the diathermaneity [diather- 
manéité | of white glass. It has so happened in our experiments: for 
if we take the natural transmissions of the white, red, orange, yellow, 
blue, indigo, and violet, and compare these with their transmissions when 
