EADIATION OF HEAT BY OASEOUS MATTEE. 
95 
bromine of course augmented the chill, and thus magnified the effect which in reality 
he was measuring. In reference to heating the glass plates by the flame, made use of 
in his experiments, I will cite a single passage from the memoir of Dr. Feanz. It refers 
to the vapour of iodine produced by throwing the substance on a heated surface in a 
vessel closed with glass plates. “The mirror,” he writes, “showed a deflection of only 
178. But as the glass plates through which the heat radiated had not yet assumed a 
temperature high enough to reduce the iodine, which had been precipitated upon them 
in crystals, to a state of vapour it was necessary to wait, and allow the radiation 
of the lamp to continue till all the iodine was driven from the bottle This shows 
how much the glass plates could be heated by the radiation of the lamp ; this heat on a 
particular occasion being sufficient to dissipate the solid iodine which had coated the 
glass plates. 
§14. 
As a dam built across a river causes a local deepening of the stream, so our atmo- 
sphere, thrown as a dam across the terrestrial rays, produces a local heightening of the 
temperature at the earth’s surface. This, of course, does not imply indefinite accumu- 
lation any more than the river dam does, the quantity lost by terrestrial radiation being, 
finally, equal to the quantity received from the sun. The chief intercepting substance 
is the aqueous vapour of the atmosphere f , the oxygen and nitrogen of which the great 
mass of the atmosphere is composed being sensibly transparent to the calorific rays. 
Were the atmosphere cleansed of its vapour, the temperature of space would be directly 
open to us; and could we under present circumstances reach an elevation where the 
amount of that vapour is insensible, we might determine the temperature of space by 
direct experiment. Colonel Steachey has written an admirable paper on the aqueous 
vapour of the atmosphere J, in which he shows that the amount of vapour diminishes 
much more speedily with the elevation than might be inferred from the law of 
Daltoiv. 
It might be possible to reach a height where, by preserving one face of a thermo- 
electric pile at the temperature of the locality, the other, protected from all terrestrial 
radiation, turned to the zenith, would assume the temperature of space in that direc- 
tion §, while the consequent galvanometric deflection would give us the means of deter- 
mining the difference in temperature between the two faces of the pile. Knowing one, 
we should therefore be able to determine the other ; knowing the temperature of the 
locahty, we could infer from it the temperature of stellar space. Many eminent writers, 
♦ “Es musste bei fortdauernder Strahlung der Lampe der Zeitpunct abgewartet werden.” 
t The mildness of an island climate must be in part due to this cause. The direct tendency of the vapour 
is to check sudden fluctuations of temperature. Where it is absent, as at the surface of the moon, such 
fluctuations must be enormous. The face turned towards the sun drinks in the solar rays without let or 
hindrance, while the radiation of the face turned from the sun pours unchecked into space. 
f Proceedings of the Eoyal Society, vol. xi. p. 182. 
§ A well of cold air would be formed within the reflector, the lowest stratum of the well sharing the 
temperature of the face of the pile. 
