68 FRAGMENTS OF SCIENCE 



to rank that y,apor as our most powerful absorber of radi- 

 ant heat. Its attenuation, however, diminishes its action. 

 I have proved that a shell of air two inches in thickness 

 surrounding our planet, and saturated with the vapor of 

 sulphuric ether, would intercept 86 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 ex- 

 tremely probable that the estimate already made of the 

 action of atmospheric vapor 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 de- 

 termined, but certainly far beyond what has hitherto been 

 imagined, for the temperature 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 sending forth calorific waves, which, on passing 

 among the 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 that occu- 

 pied our thoughts. "We have now to change the form of 

 our conception, and to figure these molecules not as ab- 

 sorbers, but as radiators, not as the recipients, but as the 

 originators of wave-motion. That is to say, we must figure 

 them vibrating, and generating in the surrounding ether 

 undulations which speed through it with the velocity of 

 light. Our object now is to inquire whether the act of 

 chemical combination, which proves so potent as regards 



