EADIATION OE HEAT BY GASEOUS MATTEE. 
87 
the very experiments devised to show conductivity showed in a very striking manner 
the existence of athermancy, or opacity to radiant heat, in the case of a considerable 
number of gases. 
The experiments on radiation, w^here obscure heat was made use of, were thus con- 
ducted. Two glass vessels, one much larger than the other, had their bottoms fused 
together; the larger one being turned upside dow, the smaller one stood upright on 
the top of it. The mouth of the larger vessel was ground down, so that it could be 
placed like an ordinary receiver on the plate of an air-pump and exhausted, while through 
proper openings different gases could be afterwards admitted into it. 
To the plate of the air-pump on which the above vessel was placed, was attached a 
thermo-electric pile with mres leading from it, through the plate, to a galvanometer ; 
the axis of the pile was vertical, one face of it being turned do-wnwards towards the 
plate, and the opposite face tuimed upwards towards the common surface of the two 
vessels which had been fused together. 
Water was placed in the uppermost vessel, and caused to boil by conducting hot steam 
through it. Its under surface became thus heated to a temperature of 100° C. But 
this under surface constituted the upper surface of the vessel underneath. This latter, 
therefore, possessed a temperatm’e of 100° C. ; and it formed the source of heat made 
use of in the experiments. 
Here Professor Wiaxus had a radiating surface of glass — a good radiator — kept at 
a constant temperatm’e by the hot water above it ; at a distance from this surface and 
turned towards it was the thermo-electric pile, defended from the radiation of the 
surface, or exposed to it, at pleasure, by the action of a moveable screen. The entire 
space between the pile and the radiating surface could either be rendered a vacuum, 
offering no resistance to the passage of the calorific rays, or else be filled by a gas the 
diathermancy of which was to be examined. 
The concurrence of the experiments made with this apparatus and those made with 
mine is, as I have stated, remarkable. Some differences, however, exist between my 
fi’iend and myself, a few remarks on which will not be without then.’ use to those who 
may afterwards enter upon this extensive field of inquiry. 
Experimenting in the ordinary way with his thermo-electric pile — using one of its 
faces only — Professor Magxus finds that air and oxygen cut off each more than 11 per 
cent, of the heat emanating from his source, while hydrogen cuts off more than 14 per 
cent. * I, on the contrary, with the most delicate means I could apply, failed to esta- 
blish the absorption of these gases by experiments made in the ordinary manner f. In 
fact it was their neutrality that drove me to devise the principle of compensation, briefly 
referred to at the commencement of this memoir. I was so particular in the experiments 
which led me to the above negative result, that if the absorption amounted to one-tenth 
of that found by Professor Magnus I do not think it could have escaped me. Nor do 
1 think that if such an action existed Melloni could have concluded that the absorption 
* Page 30, t Philosophical Transactions, 1861. 
