58 FBAGMENTS OF SCIENCE. 



12. Aqueous Vapour in relation to the Terrestrial 

 Temperatures. 



We are now fully prepared for a result which, 

 without such preparation, might appear incredible. 

 Water is, to some extent, a volatile body, and our 

 atmosphere, resting as it does upon the surface of the 

 ocean, receives from it a continual supply of aqueous 

 vapour. It would be an error to confound clouds or 

 fog or any visible mist with the vapour of water, which 

 is a perfectly impalpable gas, diffused, even on the 

 clearest days, throughout the atmosphere. Compared 

 with the great body of the air, the aqueous vapour it 

 contains is of almost infinitesimal amount, 99^ out of 

 every 100 parts of the atmosphere being composed of 

 oxygen and nitrogen. In the absence of experiment, 

 we should never think of ascribing to this scant and 

 varying constituent any important influence on terres- 

 trial radiation ; and yet its influence is far more potent 

 than that of the great body of the air. To say that on 

 a day of average humidity in England, the atmospheric 

 vapour exerts 100 times the action of the air itself, 

 would certainly be an understatement of the fact. 

 Comparing a single molecule of aqueous vapour with 

 an atom of either of the main constituents of our 

 atmosphere, I am not prepared to say how many 

 thousand times the action of the former exceeds that of 

 the latter. 



But it must be borne in mind that these large 

 numbers depend, in part, on the extreme feebleness of 

 the air; the power of aqueous vapour seems vast, 

 because that of the air with which it is compared is 

 infinitesimal. Absolutely considered, however, this 

 substance, notwithstanding its small specific gravity, 



