SECT. ii. EFFECTS OF PURE DRY AIR. 53 



may be powerful absorbers and radiators, but in their 

 strata they constitute an open sieve through which a 

 great quantity of radiant heat may pass. Ln such thin 

 strata, therefore, the vapours as used in the experi- 

 ments were generally found far less energetic than the 

 gases, while in thick strata the same vapours showed 

 an energy greatly superior to the same gases, but the 

 gases were always employed at a pressure of one atmo- 

 sphere. 



Lastly Mr. Tyndall examined the diathermancy of 

 the liquids from which his vapours were derived, and the 

 result leaves not a doubt that both absorption and 

 radiation are phenomena irrespective of aggregation. 

 If any vapour is a strong absorber and radiator, the 

 liquid from whence it comes is also a strong absorber 

 and radiator. 



Perfectly dry pure air is as pervious to light and heat 

 as a vacuum itself; consequently, if the atmosphere was 

 quite pure and dry, the rays of the sun would fall on 

 the earth with unmitigated force during the day, and 

 would be radiated back again and dissipated in space 

 during the night to the destruction of vegetation. But 

 the earth is protected from these extremes by the ab- 

 sorptive power of aqueous vapour, which is always 

 present more or less in the atmosphere ; even when the 

 air is so transparent that distant objects seem to be near, 

 it is loaded with vapour in an elastic invisible state, 

 which a change of temperature may condense into cloud 

 or precipitate in rain. 



The absorptive power of aqueous vapour was deter- 

 mined by placing tubes containing fragments of glass 

 moistened with water between the drying apparatus 

 and the experimental glass tube of the instrument, so 

 that perfectly pure dry air in passing over the wet frag- 

 ments of glass carried a portion of aqueous vapour with 

 it into the exhausted experimental tube, and the deflec- 



