216 
PROFESSOR TYNDALL ON THE ABSORPTION AND 
Table XXXIII. — Acetic Ether, 0-5 inch. 
Length < 
of Chambers. 
Sum of Absorptions. 
^Absorption of Sum. 
2-8 
46-6 
77-2 
62-9 
8-0 
41-4 
88-8 
64-6 
12*2 
37-2 
96-7 
64-2 
15*4 
34-0 
99-9 
62-4 
23'8 
25*6 
103-6 
64-7 
36*3 
13-1 
100-7 
64-8 
Means 94-5 
An inspection of the foregoing Tables discloses the fact that, in the case of vapours, 
the difference between the sum of the absorptions and the absorption of the sum is, in 
general, less than in the case of gases. This resolves itself into the proposition that for 
equal lengths, within the limits of these experiments, the sifting power of the gas is 
greater than that of the vapour. The reason of this is that the vapours are examined 
in a state of tenuity which is only g^th of that possessed by the gases. Thus, no matter 
how powerful the individual molecules may be, their distance asunder renders a thin 
layer of them a comparatively open screen. 
§3. 
The entrance of a gas into an exhausted vessel is accompanied by the generation 
of heat ; and the gas thus warmed, if a radiator, will emit the heat generated. Con- 
versely, on exhausting a vessel containing any gas, the gas is chilled, and thus an 
external body, which prior to the act of exhaustion possessed the same temperature as 
the gas within the vessel, becomes, on the first stroke of the pump, a warm body with 
reference to the gas remaining in the vessel ; and if the external body be separated from 
the cooled gas by a diathermic partition, it will radiate into the gas and become chilled 
by this radiation. It was shown in my second memoir* that this mode of warming 
and of chilling a gas or vapour furnished a practical means of determining, without any 
source of heat external to the gaseous body itself, both its radiative and absorptive 
energy. For the sake of convenience I have called the radiation and absorption of a 
gas or vapour thus dynamically heated and cooled, dynamic radiation and dynamic 
absorption. 
In illustration of the manner in which dynamic radiation may be applied in researches 
on radiant heat, I have had made, during the last half-year, a considerable number of 
experiments, some of which I will here describe. In the first place, the experimental 
tube was divided into two compartments, as in the experiments described in the fore- 
going section. The source of heat was abolished, and one end of the experimental tube 
was stopped by a plate of polished metal ; the other end was stopped by a transparent 
plate of rock-salt, while the space between the ends was divided into two compartments 
* Philosophical Transactions, Part I. 1862 ; and Philosophical Magazine, vol. sxiv. p. 337. 
