Page:Scientific Memoirs, Vol. 1 (1837).djvu/80

68 circumstance which makes it easier to compare the results than it is found to be when we are obliged to have recourse to two thermoscopes, which seldom or never possess the same degree of sensibility.

I shall now give the ratios derived from this process applied to direct heat, and to heat transmitted through several screens. The calorific effect produced each time on the black surface is represented by 100.

Thus the interposition of the rock salt has no influence on the ratio of the quantities of heat absorbed by tlie two surfaces ; but the alum affects it so strongly that the heat which has traversed a plate of this substance is much less capable than the direct heat is of being absorbed by the white surface. Colourless glass acts in a similar manner though with somewhat less energy. As to coloured glasses, their action is more feeble in proportion as their tint is less vivid. In short the greatest decrease in the absorption of the white surface is produced by the interposition of a yellow glass, and the least by the interposition of the red and the violet, and, as to each pair of plates of the same tint, the less effect is invariably derived from that in which the tint is deeper. This decrease of action which takes place in the vitreous matter in proportion as its transparency is diminished by addition of colouring substances more and more sombre, continues even when the glass loses its transparency altogether; for the plate of opake black glass is that which produces the least difference of absorption between the black and the white surfaces. It is however an exceedingly curious fact that the rays of heat in their passage through the black glass become more absorbable by the white surface than the rays issuing immediately from the lamp, so that the interposition of the black glass has on the direct heat an effect contrary to that produced on it by the interposition of the white glass. screens, but is abundantly transmitted by all the diaphanous colourless plates. It suffers no appreciable loss when the thickness of the plates is varied within certain limits. Its properties of transmission therefore bear a close resemblance to those of light and solar heat. Let us now direct our attention to the rays which issue from the last two screens. The opake bodies transmit nearly the half of them; the