Page:Scientific Memoirs, Vol. 1 (1837).djvu/64

52 The black mica and black glass then, though perfectly opake, are diathermanous, but yet only partially diathermanous, because while they allow some rays of heat to pass they intercept others.

We may see, besides, that the heat of incandescent platina and that of the flame of oil are transmitted in nearly equal quantities by these two substances. As soon as I had made my first experiments on the transmission of opake bodies I found that the rays from incandescent platina pass through a plate of black glass in a greater proportion than those from an Argand lamp. Now as it happens quite otherwise in respect to transparent glass and other diathermanous bodies, I thought at first that, in the particular case of the black glass, the variation in the quantity of heat transmitted was inversely as the temperature of the radiating source. But it was not long before I discovered my mistake; for, exposing two flakes of glass, the one colourless and the other opake, first to the direct rays of a Locatelli lamp and next to the rays that passed through a screen of common glass, I found that if the transmission through the first plate increases, as I have already stated in my first Memoir, the transmission through the second decreases. These opposite variations exhibited by the transmissions of the black and the white glass relatively to the radiations from the Argand lamp, and the incandescent platina, do not arise from any peculiar action of the calorific sources on the two bodies, but from a particular modification which the cylindrical screen or glass funnel attached to the Argand lamp produces in the calorific rays passing through it, — a modification which changes their capability of ulterior transmission and enables them to pass through the other bodies in a greater or less quantity than if they were in their natural state. We shall presently see that almost all the screens produce analogous effects.

The similarity of the action of glass and transparent bodies in general upon radiant heat to that of coloured media upon light, is established even in its most minute details by all the phænomena of transmission that we have been able to observe. For we have seen that the calorific rays from the flame of an Argand lamp lose much of their intensity while passing into the interior of a thick piece of colourless glass, and that their subsequent losses decrease in proportion as the distance from the surface at which they enter increases. Now the same thing takes place if we expose to white light any coloured transparent body, a red liquid, for instance; for in this case nearly all the rays, blue, green, yellow, &c., which enter into the composition of this light are absorbed more or less rapidly by the first layers of the liquid, and the red rays alone penetrate to a certain depth.