RADIATION. 199 



change their relative powers of absorption. Nothing could 

 more clearly prove that the act of absorption depends upon 

 the individual molecule, which equally asserts its power in 

 the liquid and the gaseous state. We may assuredly con 

 clude from the above table that the position of a vapor is 

 determined by that of its liquid. Now, at the very foot of 

 the list of liquids stands water, signalizing itself above all 

 others by its enormous power of absorption. And from 

 this fact, even if no direct experiment on the vapor of water 

 had ever been made, we should be entitled to rank that 

 vapor as the most powerful absorber of radiant heat hitherto 

 discovered. It has been proved by experiment that a shell 

 of air two inches in thickness surrounding our planet, and 

 saturated with the vapor of sulphuric ether, would intercept 

 35 per cent, of the earth s radiation. And though the 

 quantity of aqueous vapor necessary to saturate air is 

 much less than the amount of sulphuric ether vapor which 

 it can sustain, it is still extremely probable that the esti 

 mate already made of the action of atmospheric vapor 

 within 10 feet of the earth s surface, is altogether under 

 the mark ; and that we are ^indebted to this wonderful sub 

 stance, to an extent not accurately determined, but certainly 

 far beyond what has hitherto been imagined, for the tem 

 perature now existing at the surface of the globe. 



14. Reciprocity of Radiation and Absorption. 



Throughout the reflections which have hitherto occupied 

 us, the image before the mind has been that of a radiant 

 source generating calorific waves, which, on passing among 

 the scattered molecules of a gas or vapor, were intercepted 

 by those molecules in various degrees. In all cases it was 

 the transference of motion from the ether to the compara 

 tively quiescent molecules of the gas or vapor. We have 

 now to change the form of our conception, and to figure 



