60 
PEOFESSOE TYJ^DiVEL ON THE ABSOEPTION AND 
that received from the experimental tube by its anterior surface. When this was 
eliected, no matter how high the previous deflection might be, it was completely neutra- 
lized, and the needle descended to zero. 
Supposing this equality to have been established when the tube was exhausted, it is 
manifest that any gas, capable of absorbing even an extremely small proportion of radiant 
heat, would, if introduced into the tube, destroy the equilibrium of both sources. 
The second source of heat would now predominate, and a deflection of the galvanometer 
needle would be the consequence. The magnitude of this deflection would depend on 
the quantity of heat cut off by the gas, and properly reduced it became a strict measure 
of the absorption. * 
But in these experiments my fii’st source of heat stood at some distance from the 
anterior end of the tube, and the heat, previously to entering the latter, had to cross a 
space of air which \vas not the subject of examination. This air-space I wished to 
abolish, so as to allow the calorific rays to enter the gas with all the qualities which 
they possessed at the moment of emission. I first thought of soldering the end of the 
experimental tube direct to the radiating surface, thus allowing the air to come into 
direct contact with the source. But it immediately occurred to me that the intro- 
duction of cool air into the tube would lower the temperature of the source, and that I 
could never know how far the indication of my galvanometer under such circumstances 
could be regarded as a true effect of absorption ; hence I abandoned the idea of bringing 
the gases into contact with the radiating surface. 
Instead of this arrangement an independent tube, 8 inches long, and of the same 
diameter as the experimental tube, was soldered on to the radiating plate. By means of 
a screw joint, the free end of this tube was connected air-tight with the experimental 
tube. Thus a chamber, from which the air could be removed, was introduced between 
the first plate of salt and the radiating surface. Two objects were thus secured; firstly, 
my source of heat was withdrawn from the action of irregular currents of air; and, 
secondly, the radiant heat entered the tube unchanged in quality save the infinitesimal 
change due to its passage through the diathermic salt. 
To save the trouble and expense of a new Plate, I will ask permission to reprint in 
this memoir the Plate made use of in my last ; a verbal reference will in most cases be 
sufficient to indicate the changes recently introduced. S S' (Plate I.) it will be remem- 
bered represented the experimental tube, which was then made of brass polished within. 
Such a tube could not be used for any gases or vapours capable of attacking brass ; 
and though I combated this difficulty, to some extent, by blackening the tube within, 
I could never feel at ease regarding the action of the gases upon the blackening sub- 
stance. Many gases, moreover, present great difficulties on account of their affinity for 
atmospheric moistm’e. Hydrobromic and hydrochloric acid, for example, form dense 
fumes in the air, and however carefully they might have been dried, I should have been 
reluctant to base any inference on their deportment 'without actually having them under 
my eyes dm'ing experiment. 
