Page:Scientific Memoirs, Vol. 1 (1837).djvu/62

50 whether exposed to the radiations of flame, of incandescent platina, of copper heated to 390°, or of boiling water, always transmits 92 of every hundred incident rays.

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 15mm or 20mm 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 86mm. The quantity of heat transmitted by this series of flakes was considerably less than because of the great number of successive reflexions; but it was always invariable relatively to the four sources. Between these limits of thickness, therefore, rock salt really acts in respect to radiant heat just as colourless 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 transmitted the heat in a proportion less than 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 certain sources to pass through them and intercept the rays from others: they act therefore in respect to heat as coloured media act on light 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 transmission through 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 analogous 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 colourless diaphanous media on luminous rays. On this ground it may be said that the inference of Brewster was yet premature; besides, the illustrious Scotchman 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 different 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 absorbed, (Annales de Chimie et de Physique, December 1831,) and we have just seen in the preceding note that analogous phænomena are observable 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. .

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