Page:Scientific Memoirs, Vol. 1 (1837).djvu/81

Rh 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 analogous 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 differential 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 resemblance 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 calorific 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, indigo, 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 quantity 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 calorific transparencies of these plates will be increased or diminished in proportion to the variation produced in the diathermancity [diathermaméité] 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