62 FKAGMENTS OF SCIENCE. 



does not change their relative powers of absorption. 

 Nothing could more clearly prove that the act of ab- 

 sorption depends upon the individual molecule, which 

 equally asserts its power in the liquid and the gaseous 

 state. We may safely conclude from the above table 

 that the position of a vapour is determined by that of 

 its liquid. Now at the very foot of the list of liquids 

 stands water, signalising itself above all others by its 

 enormous power of absorption. And from this fact, 

 even if no direct experiment on the vapour of water 

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

 vapour as our most powerful absorber of radiant heat. 

 Its attenuation, however, diminishes its action. I have 

 proved that a shell of air two inches in thickness sur- 

 rounding our planet, and saturated with the vapour of 

 sulphuric sether, would intercept 35 per cent, of the 

 earth's radiation. And though the quantity of aqueous 

 vapour necessary to saturate air is much less than the 

 amount of sulphuric sether vapour which it can sustain, 

 it is still extremely probable that the estimate already 

 made of the action of atmospheric vapour within 10 

 feet of the earth's surface, is under the mark ; and that 

 we are indebted to this wonderful substance, 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 oc- 

 cupied us, the image before the mind has been that of 

 a radiant source sending forth calorific waves, which on 

 passing among the molecules of a gas or vapour were 

 intercepted by those molecules in various degrees. In 



