5^ M. MELLOXI OX THE IMMEDIATE TRANSMISSION' 



The black mica and black glass then, tkoiiyh jyerfectly opahe, are dia- 

 Ikermanous, but yet only partially diatheinianous, because while they al- 

 low 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 trans- 

 mission of opake bodies I found that tlie rays from incandescent pla- 

 tina pass througli 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 ra- 

 diating 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 aLocatelli 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 Me- 

 moir, 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 incan- 

 descent 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 pro- 

 duces 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 phsenomena of transmission that 

 we have been able to observe. For we have seen that tlie 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, yel- 

 low, &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. 



* Bulletin de la Societe Philomatiqiie, July 1S33, 



