ON THE ABSORPTION AND RADIATION OF HEAT. 355 
notion hitherto held, that gases are almost perfectly diather- 
manous (transparent for heat). 
We find ammonia even a more powerful absorber than 
olefiant gas, indeed, it is almost black to rays of heat emanating 
from a source of low temperature. When the gas was in the 
tube, the interposition of a double metallic screen between the 
pile and source augmented the deflection very slightly. 
In all experiments hitherto mentioned, gas at the tension of 
one atmosphere was employed. At lower tensions, the absorp- 
tion becomes less, but the results obtained with different gases 
are still more striking than those arrived at under ordinary 
pressure. One, two, or more measures of olefiant gas were 
allowed to diffuse through the experimental tube S S', and the 
consequent deflections of the needle observed. 
By this means we arrive at the result that at low tensions the 
absorptions are proportional to the quantity of gas admitted 
into the experimental tube. One measure of olefiant gas 
absorbs a quantity of heat expressed by the number 2*2 and 
10 measures of gas 2'2 x 10 = 22 of heat. The same rule 
holds good for other gases; but this proportion does not 
obtain when large quantities are employed. 
Hence, the absorptions are only proportional to the quantity 
of the gas when small quantities are employed. 
If we assume oxygen, nitrogen, air, and hydrogen to absorb 
at one-inch tension one-thirtieth of the quantity of radiant 
heat which they do absorb at 30-inch tension, an assumption 
which cannot be far from the truth, and take the amount 
of heat absorbed by air at one-inch tension as unity, we obtain 
the extraordinary result, that for tensions of one inch of 
mercury the absorption of ammonia is over seven thousand 
times, the absorption of olefiant gas seven thousand nine 
hundred and fifty times, whilst the action of sulphurous acid 
is eight thousand eight hundred times that of air. 
Melloni found in the case of solids and liquids, that trans- 
parency for light and transmission of heat are independent of 
each other. Dark brown chloride of sulphur transmitted 63 
per cent., and colourless transparent water only 11 per cent, 
of radiant heat from the same source. Professor Tyndall 
observed analogous comportment in gases. Hydrobromic 
acid is transparent and colourless like air, bromine vapour 
of a dark brown colour, yet the former absorbs nearly eight 
times as much heat as the latter. 
The elements are all bad absorbers, whereas the superior 
absorbers are all compound bodies. In general, it may be 
said that, everything else remaining the same, the absorbing 
energy of a molecule increases when its structure becomes 
more complicated. Thus carbonic acid is a better absorber 
