50 M. MET.LONI ON THE IMMEDIATE TRANSMISSION 



Avhether exposed to the radiations of flame, of incandescent platina, of 

 copper heated to 390°, or of boiling water, always transmits 92 of eveiy 

 hundred incident rajs. 



The same constancy of transmission is observable when we operate on 

 sources of a temperature yet lower than that of boiling water; such, 

 for instance, as vessels containing this liquid heated to 40° or 50°. It is 

 observable also when we employ pieces of rock salt 15""" or 20'""" thick. 

 I have placed all the flakes of salt that I could dispose of side by side, 

 so that the thickness of them all amounted to 86'""". The quantity of 

 heat transmitted by this series of flakes was considerably less than tVtt> 

 because of the great number of successive reflexions ; but it Avas always 

 invariable relatively to the four sources. Between these limits of thick- 

 ness, therefore, rock salt really acts in respect to radiant heat just as co- 

 lourless glass and colourless diaphanous bodies in general act in respect to 

 light. 



This being premised, it is clear that if each substance contained in 

 the table acted like the second specimen of rock salt, that is, if it trans- 

 mitted the heat in a proportion less than tVt> but always the same for 

 each of the four sources, all these substances would be to radiant heat 

 that which diaphanous bodies more or less dusky are to light. But they 

 allow the rays from cei-tain sources to pass through them and intercept 

 the rays from others : they act therefore in respect to heat as coloured 

 media act on light*. 



What do we find when we expose the same coloured glass successively 



* It appears that Sir David Brewster had lately arrived at the same conclusion 

 by means only of the experiments of Delaroche and Seebeck on the transmis- 

 sion througli glass and on the distribution of heat in the solar spectra produced 

 with different prisms, (See Report of the First and Second Meetings of the 

 British Association for the Advancement of Science. London, 1833, p. 294.) But 

 these experiments did not prove that the rays in passing through the different 

 bodies suffer a real infernal absorption analnffoits to that which light suffers: 

 above all, they were far from proving that this absorptive force, varying in each 

 substance according to the temperature of the calorific source, could, in some 

 particular cases, become constant, and in all respects similar to the action of co- 

 lourless diaphanous media on luminous rays. On this ground it may be said 

 that the inference of Brewster was yet premature ; besides, the illustrious Scotch- 

 man rested his conjectures on the erroneous supposition that water has the same 

 absorbent force in respect to all sorts of calorific rays. Experiment indeed leads 

 to the opposite conclusion, as we have already proved in respect to solar heat 

 by the different action of a layer of water on the temperatures distributed in each 

 band of the solar spectrum ; an action so widely different relatively to two dif- 

 ferent rays that all the heat of the violet light passes through the liquid without 

 suffering any sensible diminution, while the nonluminous heat of the isothermal 

 band is totally absorl ad, {AnnalesdeChimie et de Physique, December 1831,) and 

 we have just seen in the preceding note that analogous phfenomena are obser- 

 vable in the radiations from terrestrial sources also ; for a mass of water some 

 millimetres in thickness intercepts all but a very small portion of the radiant 

 heat issuing from flame and the whole of those rays that issue from any other 

 source. 



