az M. MELLONI ON THE IMMEDIATE TRANSMISSION 
The black mica and black glass then, though perfectly opake, are dia- 
thermanous, but yet only partially diathermanous, 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 the rays from incandescent pla- 
tina 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 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 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 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 pheenomena 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, 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 ae 
alone penetrate to a certain depth. 
* Bulletin de la Société Philomatique, July 1838. 
