OF RADIANT HKAT THROUGH DIFFERENT BODIli.S. fif) 



diaphanous substances intercept them in very different quantities, and 

 the portions transmitted are considerably diminished by increasing the 

 thickness of the flakes. Thus, the rays emerging from the black or the 

 green glasses are in respect to their properties of transmission as it were 

 antagonist to the preceding, and analogous to those of the direct heat 

 of the flame though still more decidedly marked, for they are almost 

 completely absorbed by bodies possessing the greatest transparency. 



I have availed myself of these last facts for the purpose of proving by 

 a very simple process that solar light contains some calorific rays analo- 

 gous to those which compose the radiant heat of terrestrial sources. 

 With this vie\v' I introduced a solar ray into a dark room through an 

 aperture having a screen of green glass as a stopper. To the light 

 transmitted I exposed one of the blackened balls of a very delicate dif- 

 ferential thermometer. The liquid column descended several degrees. 

 I now placed quite close to the mouth of the aperture a thin plate 

 of colourless glass ; the liquid came back a little, but the retrograde 

 movement became more decided when I interposed instead of the thin 

 glass a plate of greater thickness. I took away the m hite glass and put 

 in its place a plate of rock salt : the column was forcibly driven back, 

 but reascended very nearly to its original position when I substituted 

 for the salt a jjlate of very limpid alum. It is clear therefore that 

 amongst the calorific raj's of the sun there are some which have a re- 

 semblance to terrestrial heat. On the other hand we have seen that the 

 rays from terrestrial flame which traverse a flake of alum suffer, like 

 solar heat, only a very slight diminution in passing through glass and 

 other diaphanous substances. Whence we infer that amongst the ca- 

 lorific rays from flame some are found similar to the heat of the sun. 

 Tlie differences observed between solar and terrestrial heat, as to their 

 properties of transmission, are therefore to be attributed merely to the 

 mixture, in different projjortions, of several species of rays. 



But, to return to the heat emerging from the screens exposed to the 

 radiation of the lamp. We have said that the red, orange, yellow, blue, in- 

 digo, and violet matters which enter into the composition of the coloured 

 glasses, act upon radiant heat as the black substances introduced into a 

 coloured medium act relatively to light ; that is, they diminish the quan- 

 tity of heat transmitted by the glass without altering its diathermancy 

 \^diathermansie'\. This proposition being admitted, it will necessarily 

 follow, when rays ofdifferent species, such as issue from the five screens 

 contained in the table, fall on a series of coloured glasses, that the ca- 

 lorific transparencies of these plates will be increased or diminished in 

 proportion to the variation i)roduced in the diathcrmaneity [^diather- 

 mnne.ile'\ of white glass. It lias so happened in our experiments : for 

 if we take the natural transmissions of the white, red, orange, yellow, 

 blue, indigo, and violet, and compare these with their transmissions when 



