Radiation of Heat by Gaseous Matter. 433 



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 atmosphere, thrown as a dam across the ter- 

 restrial rays, produces a local heightening of the temperature at 

 the earth's surface. This, of course, does not imply indefinite 

 accumulation, 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 t, 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; aud could we under present circumstances reach an eleva- 

 tion where the amount of that vapour is insensible, we might 

 determine the temperature of space by direct experiment. Colonel 

 Strachey has written an admirable paper on the aqueous vapour 

 of the atmosphere J, in which he shews that the amount of vapour 

 diminishes much more speedily with the elevation than might 

 be inferred from the law of Dalton. 



It might be possible to reach a height where, by preserving 

 one face of a thermo-electric pile at the temperature of the loca- 

 lity, 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 determining 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 

 locality, we could infer from it the temperature of stellar space. 



* " Es musste bei fortdauerader Strahlung der Lampe den 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 tem- 

 perature. Where it is absent, as at the surface of the moon, such fluctua- 

 tions 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. 



% Phil. Mag. S. 4. vol. xxiii. p. 152. 



§ 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. 



