Absorption of Heat on the Formation of Dew, 117 



vapours, since from their easy oxidability it was not possible to 

 obtain certain results. From its un changeability, carbonic acid 

 affords a more certain criterion for judging the radiating power 

 of vapours than all other vapours. Yet a comparison with it 

 shows very decisively that aqueous vapour in its transparent con- 

 dition has scarcely greater absorptive capacity for heat than dry 

 atmospheric air, and hence that the absorptive capacity of both 

 differs little from each other. 



But I think these experiments were not needed. A well-known 

 phenomenon, which depends on the radiation of heat, furnishes a 

 more striking proof of the small absorptive capacity of aqueous 

 vapour than all experiments in the laboratory. If aqueous vapour 

 were in fact so good an absorbent of heat as Professor Tyndall 

 maintains, dew could never be formed ; for the aqueous vapour, 

 which is indispensable for dew, would at the same time form a 

 covering over the surface of the earth and prevent its radiation. 

 But just where the atmosphere is particularly rich in water, in 

 the tropics, is dew principally formed; and those regions would 

 be devoid of all fertility were it not that moisture is imparted to 

 the plants by dew. Should it be urged that the vapour absorbs 

 heat indeed, but radiates a portion only back to the earth while 

 the greater part goes to the higher regions of the atmosphere, 

 this process of partial radiation would repeat itself from layer to 

 layer, and hence the temperature in different layers must dimi- 

 nish with the height. This, however, is notoriously not the case 

 in the formation of dew : the temperature merely sinks near the 

 good radiating surface of the earth ; and a few feet above, it is not 

 lower than over a badly-radiating place which is not covered with 

 dew. Further, a cooling would be impossible, since all layers of 

 the atmosphere, just as they radiate away part of their heat from 

 the earth, send another portion back to it. If aqueous vapour 

 possessed so great an absorptive capacity as Professor Tyndall 

 ascribes to it, extremely little of the radiating heat could reach 

 the clouds, since the enormous layer of vapour extending to them 

 would entirely absorb it. It would then be inexplicable that 

 clouds prevent dew. Since Wells's memorable investigations, it 

 is generally assumed that thermal rays reach the clouds almost 

 undiminished, and are thence reflected back to the earth j if 

 there were no such almost unhindered transmission through 

 moist air, the clouds at their great distance could not hinder the 

 radiation as does a board or any other solid body at a small dis- 

 tance from the earth. 



The conclusions which Professor Frankland deduces for the 

 glacial epoch from the great absorptive capacity of aqueous va- 

 pour, and Professor Tyndall for certain climatic phenomena, 

 remain unchanged if nebulous be substituted for actual vapour. 



