222 Mr. B. Stewart on the Radiation and Absorption of Gases. 



and reduce it to that quality which is passed without absorption 

 by atmospheric air. 



On the whole, therefore, if we consider the above hypothesis 

 as at all probable, such an arrangement cannot, I think, be 

 viewed as a favourable one for exhibiting the absorptive power 

 of atmospheric air, the effect being apparently to diminish this 

 property of air. 



Let us now, on the other hand, reflect how the radiation of 

 aqueous vapour is influenced by the same arrangement. Accord- 

 ing to hypothesis, the heat radiated and absorbed by aqueous 

 vapour possesses a comparatively high refrangibility. A consi- 

 derable portion of it will therefore be included in the radiation 

 between 60° F. and 212° F., and this will not be stopped by 

 rock-salt. The effect of the arrangement will therefore be appa- 

 rently to increase the radiation of aqueous vapour, and also of 

 many of those vapours which possess high boiling-points. 



Finally, suppose that we introduce into the vacuum-tube a 

 mixture of air and of a vapour possessing a high boiling-point. 

 Here it is evident that while little or no heat will be absorbed 

 by the first of these substances, owing to the peculiarity of the 

 arrangement, a great deal will be absorbed by the second, and 

 the result will be that air plus vapour will appear much more 

 athermanous than air alone. 



Owing to these considerations, I should hesitate to conclude 

 that the results hitherto obtained regarding the radiative and 

 absorptive powers of gases and vapours afford ground for that 

 hypothesis which asserts that the absorptive and radiative powers 

 of simple bodies are less than the corresponding qualities of com- 

 pound bodies. But, inasmuch as aqueous vapour would appear 

 to absorb a different description of heat from atmospheric air, 

 the very important meteorological conclusion deduced by Prof. 

 Tyndall would still appear to hold good on the present hypo- 

 thesis ; that is to say, air with vapour will probably stop more 

 of the heat radiated by the earth than air without vapour. 



It is even possible that there may be an additional safeguard 

 against the cooling of our globe by radiation into space, in the 

 fact that when the temperature falls very low and there is 

 scarcely any vapour in the air, the heat radiated is then of a de- 

 scription which is stopped by dry air, which would thus serve as 

 a cloak or covering, performing the same office as that which is 

 performed at a higher temperature by air loaded with aqueous 

 vapour. 



Kew Observatory, 

 August 19, 1863. 



