EADIATION OF HEAT BY OASES AND VAPOUES. 
•7 
U-tube containing fragments of pumice-stone, moistened with strong caustic potash ; 
and Z is a second similar tube, containing fragments of pumice-stone wetted with strong 
sulphuric acid. When drying only was aimed at, the potash tube was suppressed. When, 
on the contrary, as in the case of atmospheric air, both moisture and carbonic acid were 
to be removed, the potash tube was included. G G is a holder from which the gas to 
be experimented with was sent through the drying tubes, and thence through the pipe 
p p into the experimental tube S S'. The appendage at M and the arrangement at O O 
may for the present be disregarded ; I shall refer to them particularly by and by. 
The mode of proceeding was as follows : — The tube S S' and the chamber F being 
exhausted as perfectly as possible, the connexion between them was intercepted by shut- 
ting off the cocks m, m!. The rays from the interior blackened surface of the cube C 
passed first across the vacuum F, then through the plate of rock-salt S, traversed the 
experimental tube, crossed the second plate S', and being concentrated by the anterior 
conical reflector, impinged upon the adjacent face of the pile P. Meanwhile the rays 
from the hot cube C' fell upon the opposite face of the pile, and the position of the 
galvanometer needle declared at once which source was predominant. A movement of 
the screen H back or forward with the hand sufficed to establish an approximate equality ; 
but to make the radiations perfectly equal, and thus bring the needle exactly to 0°, the 
fine motion of the screw above referred to was necessary. The needle being at 0°, the 
gas to be examined was admitted into the tube ; passing, in the first place, through the 
drying apparatus. Any required quantity of the gas may be admitted ; and here expe- 
riments on gases and vapours enjoy an advantage over those with liquids and solids, 
namely, the capability of changing the density at pleasure. When the required quan- 
tity of gas had been admitted, the galvanometer was observed, and from the deflection of 
its needle the absorption y/as accurately determined. 
Up to about its 36th degree, the degrees of my galvanometer are all equal in value ; 
that is to say, it requires the same amount of heat to move the needle from 1° to 2° as 
to move it from 35° to 36°. Beyond this limit the degrees are equivalent to larger 
amounts of heat. The. instrument was accurately calibrated by the method recommended 
by Melloni (Thermochrose, p. 59), so that the precise value of its larger deflections are 
at once obtained by reference to a table. Up to the 36th degree, therefore, the simple 
deflections may be regarded as the expression of the absorption, but beyond this the 
absorption equivalent to any deflection is obtained from the table of calibration. 
The air of the laboratory, freed from its moisture and carbonic acid, and permitted to 
enter until the tube was filled, produced a deflection of about 
1 °. 
obtained from chlorate of potash and peroxide of manganese produced a 
deflection of about 
1 °. 
One specimen of nitrogen, obtained from the decomposition of nitrate of potash, pro- 
