Radiant Heat by Dry and by Moist Air. 



251 



Outer tubes. 



Inner tubes. 



Scale-reading. 



Difference. 







millims. 



rnillirns. 



Drv air 



Moist air 



550 



28 



Moist air 



Drv air 



580 



Dry air 



Moist air 



555 



30 



Moist air 



Dry air 



590 



Moist air 



Dry air 



60G 





Dry air 



Moist air 



584 



24 



Moist air 



Dry air 



610 





Moist air 



Dry air 



512 





Dry air 



Moist air 



491 



26 



Moist air 



Drv air 



522 





Moist air 



Dry air 



517 





Dry air 



Moist air 



493 



25 



Moist air 



Drv air 



519 





Coal-gas 



Dry air 



738 



418 



Dry air 



Coal-gas 



320 



In these experiments the temperature was 19° C. It follows 

 therefrom that moist air absorbs dark rays of heat more strongly 

 than dry air ; and by comparing the first differences with those 

 given in the last experiment, where coal-gas and dry air were 

 alternated with each other, it will at once be seen that the sub- 

 stitution of coal-gas in place of a stratum of dry air 45 centims. 

 in thickness, causes an absorption about fifteen times as great 

 as did the substitution of air saturated with aqueous vapour at 

 19° in place of an equally thick stratum of dry air. 



This result, however, is only an approximate one ; and not- 

 withstanding its agreement with results previously obtained, it 

 ought, strictly speaking, not to be compared therewith, because, 

 as Professor Tyndall has shown, the absorption by coal-gas is not 

 proportional to the thickness of the stratum passed through, but 

 is much stronger in the first strata traversed than in the subse- 

 quent ones. 



At all events, in these and in the former experiments no 

 question can be raised as to whether a partial condensation of 

 aqueous vapour or formation of dew may not have caused an in- 

 creased absorption. It was the air proceeding from the bottom 

 of the room which was always introduced into the apparatus; it 

 must consequently have always had a lower temperature than 

 the tubes standing above it, which, moreover, during the experi- 

 ment were heated by radiation. Besides this, as already men- 

 tioned, unchanged results were obtained when the gases, before 

 entering the tubes, were led through a water-bath whose tempe- 

 rature was lower than that of the air in the room. 



It was by this last method that I exhibited objectively the dif- 

 ference between the absorption of heat by dry and by moist air, 

 as well as by the ordinary air of a room, by coal-gas, and by the 



