RADIATION. 195 



lavender GO, that of rosemary 74, while that of aniseseed 

 amounts to 372. It would be idle to speculate on the 

 quantities of matter concerned in these actions. 



12. Aqueous Vapor in relation to the Terrestrial 

 Temperatures. 71 



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 vapor. It would be an error 

 to confound clouds or fog or any visible mist with the va- 

 por of water : this vapor is a perfectly impalpable gas, dif- 

 fused, even on the clearest days, throughout the atmosphere. 

 Compared with the great body of the air, the aqueous vapor 

 it contains is of almost infinitesimal amount, 99^ out of 

 every 100 parts of the atmosphere being composed of oxy- 

 gen and nitrogen. In the absence of experiment, we should 

 never think of ascribing to this scant and varying constitu- 

 ent any important influence on terrestrial 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 vapor exerts 100 times the 

 action of the air itself, would certainly be an understate- 

 ment of the fact. The peculiar qualities of this vapor, and 

 the circumstance that at ordinary temperatures it is very 

 near its point of condensation, render the results which it 

 yields in the apparatus already described, less than the 

 truth ; and I am not prepared to say that the absorption by 

 this substance is not 200 times that of the air in which it is 

 diffused. Comparing a single molecule of aqueous vapor 

 with an atom of either of the main constituents of our at- 

 mosphere, I am not prepared to say how many thousand 

 times the action of the former exceeds that of the latter. 



1 See Note at the end of this Lecture. 



