EADIATIOX OF HEAT BY GASES AHD VAPOUES. 
29 
to assume alterations in the density and height of the atmosphere, to account for 
different amounts of heat being preserved to the earth at different times ; a slight 
change in its variable constituents would suffice for this. Such changes in fact may 
have produced all the mutations of climate which the researches of geologists reveal. 
However this may be, the facts above cited remain; they constitute true causes, the 
extent alone of the operation remaining doubtful. 
The measurements recorded in the foregoing pages constitute only a fraction of those 
actually made ; but they fulfil the object of the present portion of the inquiry. They 
establish the existence of enormous differences among colourless gases and vapours as 
to their action upon radiant heat ; and they also show, that when the quantities are suf- 
ficiently small, the absorption in the case of each particular vapour is exactly proportional 
to the density. 
These experiments furnish us with purer cases of molecular action than have been 
hitherto attained in experiments of this nature. In both solids and liquids the 
cohesion of the particles is implicated ; they mutually control and limit each other. 
A certain action over and above that which belongs to them separately, comes into play 
and emban-asses our conceptions. But in the cases above recorded the molecules are 
perfectly free, and we fix upon them indi\fidually the effects which the experiments 
exhibit. Thus the mind’s eye is directed more firmly than ever on those distinctive 
physical qualities whereby a ray of heat is stopped by one molecule and unimpeded by 
another. 
^ 9. Badiation of Heat by Gases. 
It is known that the quantity of light emitted by a flame depends chiefly on the 
incandescence of solid matter; the brightness of an ignited jet of ordinary gas, for 
example, being chiefly due to the solid particles of carbon liberated in the flame. 
Melloxi drew a parallel between this action and that of radiant heat. He found the 
radiation from his alcohol lamp greatly augmented by plunging a spiral of platinum 
wire into the flame. He also found that a bundle of wire placed in the current of hot 
air ascending from an argand chimney gave a copious radiation, while when the wire 
was withdrawn no trace of radiant heat could be detected by his apparatus. He con- 
cluded from this experiment that air possesses the power of radiation in so feeble a 
degree, that our best thermoscopic instruments fail to detect this power *. 
These are the only experiments hitherto published upon this subject ; and I have 
now to record those which have been made in connexion with the present inquiry. The 
pile furnished with its conical reflector was placed upon a stand, with a screen of 
polished tin in front of it. An alcohol lamp was placed behind the screen, so that its 
flame was entirely hidden by the latter ; on rising above the screen, the gaseous column 
radiated its heat against the pile and produced a considerable deflection. The same 
effect was produced when a candle or an ordinary jet of gas was substituted for the 
alcohol lamp. 
* La Thermoclirose, p. 94. 
