He M. MELLONI ON THE IMMEDIATE TRANSMISSION 
as the distance from the surface of entrance increases; but the diminution 
becomes less and less perceptible, so that it must become invariable when 
the rays have penetrated to a certain depth. This is precisely what 
happens to a pencil of ordinary light when it enters a coloured medium; 
for, those rays that are of a colour different from that of the medium 
being extinguished in the first layers, the losses of intensity sustained 
by the luminous pencil are at first very great, but they afterwards be- 
come gradually less and are at last very small, but constant when the 
only rays remaining are those of the same colour as the medium. 
In fine the successive transmissions through heterogeneous screens 
furnish a third proof of the analogy which the action of diathermanous 
bodies on radiant heat bears to that of coloured media on light. The 
luminous rays issuing from a coloured plate either pass in abundance 
through a second coloured plate or undergo in it a powerful absorption 
according to the greater or less analogy of the colour of the second to 
that of the first plate. Now we observe facts perfectly similar to this in 
the successive transmission of radiant heat through screens of different 
kinds. And in this case too the rock salt acts in respect to the other 
bodies as it does in the case of rays emanating from sources of different 
temperatures. A given plate, if it be of rock salt, being successively 
exposed to calorific radiations of the same force emerging from different 
screens, transmits a constant quantity of heat ; ifthe plate be of any other 
diathermanous substance the quantity transmitted will be variable. 
There is therefore but one colourless and diaphanous body that really 
acts in the same manner on luminous and calorific rays. All other 
diaphanous bodies besides this indiscriminately suffer all kinds of light 
to pass through them, but of the rays of heat they allow some to pass 
while they absorb others : thus we discover in this one substance a real 
calorific coloration, to which, as it is invisible, and therefore totally di- 
stinct from coloration properly so called, we have given the name of dia- 
thermancy. 
The colours introduced into a diaphanous medium always diminish 
its diathermancy in a greater or a less degree, without communicating 
to it any tendency to arrest certain calorific rays rather than others: they 
affect the transmission of radiant heat as dusky bodies affect the transmis- 
sion of light. There is, it is true, an exception to be made in respect to 
green and opake black, at least in certain kinds of coloured glass. But 
these two colouring matters appear, in this case, to do no more than 
modify the quality to which we give the name of diathermancy, and 
which, as we have already seen, is totally independent of coloration. < - 
The quantity of radiant heat which passes through polarizing plates 
of tourmaline is not affected by any change made in the angle at which 
their axes of crystallization are made to cross one another. Rays of 
heat are therefore not polarized in this mode of transmission and are in 
