Radiant Heat by Dry and by Moist Air. 255 



that, notwithstanding the negative results furnished by Magnus's 

 method, I maintain that the greater absorptive power of moist air, 

 as compared with dry, has been fully established by the experi- 

 ments made according toTyndalPs method; and I am of opinion 

 that meteorologists may without hesitation accept this new fact in 

 their endeavours to explain phenomena which hitherto have re- 

 mained more or less enigmatical. It appears to me to be desi- 

 rable, however, to determine still more accurately the relative 

 values of these two absorptions. 



Appendix. 



After the above had already been sent to press, I received the 

 April Number of PoggendorfFs Annalen, wherein Professor Mag- 

 nus, in a paper " On the Influence of the Absorption of Heat on 

 the Formation of Dew"*, seeks to decide the above-disputed point 

 relative to the absorption by dry and by moist air, by comparing 

 experimentally the powers of emission of heated dry and moist 

 air, and afterwards employing the principle of the equality of the 

 ratio, in all bodies, between the powers of emission and absorption. 

 From this comparison he found that air, saturated with aqueous 

 vapour at the ordinary temperature of 15° C, emitted when 

 heated to 200° only from two to three times as much heat as dry 

 air, and that air passed through water (durchWasser gestrichen) 

 at a temperature of from 60° to 80° C. radiated only from six to 

 seven times as much heat as dry air; he found, however, that 

 dry carbonic acid and ordinary coal-gas radiated from thirty to 

 forty times as energetically as dry air. 



In my opinion, the disputed question is not only left un- 

 decided by these experiments, but is rendered thereby more 

 complicated. In the first place, the results of these experi- 

 ments are in contradiction to the statements of Professor Frank- 

 land, who on a former occasion stated that with a similar ap- 

 paratus he had obtained a very strong radiation from heated 

 aqueous vapour, as compared with that from dry air. Even 

 if we assume, with Professor Magnus, that condensed vapour 

 had here been produced, and that this had been the chief cause 

 of the increased radiation of aqueous vapour, another pecu- 

 liar divergence from the results of Professor TyndalFs and my 

 own experiments still exists; for, on the one hand, it follows 

 from the experiments of Tyndall that at ordinary temperatures 

 air saturated with aqueous vapour absorbs heat from thirty to 

 forty times as strongly as dry air does, whilst, according to Mag- 

 nus, the latter radiates only from one-half to two-thirds as much as 

 the former ; on the other hand, however, Professor TyndalPs and 

 my own measurements accord in giving a sixteen times as great 

 * See Phil. Mag. for August 1866. 



