50 M. MELLONI ON THE IMMEDIATE TRANSMISSION 
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 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 +o, 
because of the great number of successive reflexions; but it was 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 +> 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*. 
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 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 internal 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 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 absorbed, (Annales de Chimie et de Physique, December 1831,) and 
we have just seen in the preceding note that analogous phenomena 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. 
