RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
299 
absorption, and indicating cold, but to carry the needle up to 50° on the side of heat. 
So likewise, when the experimental tube was filled with mixed air and vapour, the 
needle being at 0°, a stroke of the pump, though opening a freer passage for the rays 
from the source, caused a deflection indicative not of heat, but of cold. Here, the 
vapour within the tube being chilled by the dilatation of the air, the pile radiated its 
uncompensated warmth into the vapour and produced the observed deflection. 
Such observations suggested a new means of demonstrating the absorption and 
radiation of heat by gases and vapours. Abandoning all external sources of heat, and 
permitting the various gases already examined to enter the experimental tube at a 
common velocity, they became self heated and radiated against the pile. Their 
radiation, thus determined, corresponded exactly with the results obtained when 
heated columns of these gases were permitted to rise freely in the atmosphere. 
Both the radiation and absorption of vapours were determined in the same manner. 
The external source of heat was abandoned, and a measured quantity of every vapour 
was introduced into the experimental tube. Through an orifice of fixed dimensions 
dry air was then permitted to enter the tube, where the destruction of its vis viva 
raised its temperature. The heated air warmed the vapour, which in its turn 
radiated the heat imparted to it against the pile. The deflection of the galvanometer 
declared the strength of this radiation. Absorption was determined by permitting 
the mixed air and vapour to dilate by a measured quantity, the pile being here the 
warm body, and the chilled vapour the absorbent. The order in which the vapours 
stood as regards absorption was here exactly the order of their radiation, while both 
absorption and radiation, thus determined, agreed with the results obtained by sending 
the rays from an external source of heat through the pure vapours in the experimental 
tube. 
What has been called “ vapour-hesion,” whereby liquid films are produced, has been 
supposed to play a dominant part in my experiments. But it can hardly be imagined 
that an irregular action of this kind could produce results of such precision and con¬ 
sistency as those here recorded. Such results are, in my opinion, only compatible with 
the conclusion that the veritable radiators and absorbers are the molecules of the 
vapours. Apart from all experiment, the notion that vapours must act in this way 
comes commended to us by the proved, and conceded, deportment of gases. It would 
be unreasonable to admit that a compound gaseous molecule is active, and at the 
Same time to affirm that a compound vaporous molecule is inert. 
This hypothesis of liquid films formed on the interior surface of the experimental 
tube, and on the plates of rocksalt, becomes, I think, more embarrassed as we proceed. 
It depends on the unproved assumption that liquids possess powers of absorption 
which are denied to their vapours. To water and brine, for instance, Magnus largely 
concedes such powers, but not to aqueous vapour. That the state of aggregation 
exerts an influence is not denied, but that it is here the dominant factor is open to 
doubt. To admit this would be to concede that the seat of absorption is the molecule 
2 q 2 
