RADIATION or HEAT BY OASEODS MATTER. 
75 
above surmised took place. The salt remained perfectly transparent while in contact 
with the vapour. 
Some of the experiments recorded in the Bakerian Lecture for this year (1860) had 
taught me that the dynamic heating of the air when it entered the exhausted tube was 
sufficient to produce a very sensible radiation on the part of any powerful vapour con- 
tained within the tube, but I was slow to believe that the enormous effect above 
described could be thus accounted for. My first care was to determine the difference of 
temperature between a thermometer placed within the tube at the end farthest from 
the source, and one placed without it. I then examined, by an extremely sensitive 
thermometer, the increase of temperature produced by the admission of dry air into the 
tube, and the decrease consequent on pumping out ; and found the former to be a con- 
siderable fraction of the total heat transmitted from the source. Could it be that the 
heat thus imparted to the alcohol and ether vapours, and radiated by them against the 
adjacent face of the pile, was more than sufficient to make good the loss by absorp- 
tion 1 The experimentum crucis at once suggested itself here. If the effects observed 
be due to the dynamic heating of the ah’, we ought to obtain them even when the 
sources of heat made use of in the experiments are entirely abolished ; and we should 
thus arrive at the solution of the novel, and at first sight utterly paradoxical problem. 
To determine the radiation and absorption of gases and vapours loithout any source of heat 
external to the gaseous body itself 
For the sake of bre'vity, I will call the heating of gas by its admission into a vacuum, 
the dynamic heating of the gas ; and the chilling accompanying its pumping out, dynamic 
chilling. It would also contribute to bre\ity if I were allowed to call the radiation and 
absorption of the gaseous body, consequent on such heating and chilling, dynamic radi- 
ation and dynamic absorption.^ though I fear the terms are not unobjectionable. 
§ 7. On Dynamic Radiation and Absorption. 
Both the source of heat and the compensating cube were dispensed with, and the 
thermo-electric pile was presented to the end of the cold experimental tube. By a little 
management, the slight inequality of radiation against both faces of the pile, arising 
from differences in the various parts of the laboratory, was obliterated, and the needle of 
the galvanometer thus brought to 0°. 
The vapours were admitted in the manner already described, until a tension of 0’5 of an 
inch was obtained. The air was then allowed to enter through a drying apparatus by an 
orifice of a constant magnitude. Two stopcocks, in fact, were introduced between the dry- 
ing-tube and the experimental tube ; one of these was kept partially turned on, and formed 
the gauge for the admission of the air. When the tube was to be exhausted, the second 
stopcock was turned quite off. When the tube was to be filled, this stopcock was turned 
full on ; but the gauge-cock was never touched during the entire series of experiments. 
Before, however, the mode of experiment was thus strictly arranged, a few preliminary 
trials gave me the following results : — 
L 2 
