72 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. Tliis is precisely what 

 happens to a pencil of ordinaiy light when it enters a coloured medium ; 

 for, those rays that are of a colour diflereiit 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 i-adiant 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 ; if the 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 plready 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 anotiier. Rays of 

 heat are therefore not polarized in this mode of transmission and are in 



