46 FRAGMENTS OF SCIENCE. 



the same. We have liberated the molecules from the 

 bonds which trammel them more or less jhi a liquid condi- 

 tion; but this change in their state of aggregation does not 

 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 safely 

 conclude 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 zvater, 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 

 \liat vapor 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 surround- 

 ing 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 prob- 

 able 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 wonder- 

 ful substance, to an extent not accurately determined, 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 comparatively 

 quiescent molecules of the gas or vapor that occupied our 

 thoughts. We have now to change the form of our con- 

 ception, and to figure these molecules not as absorbers but 

 as radiators, not as the recipients but as the originators of 

 wave-motion. That is to say, we must figure them vibrat- 

 ing, and generating in the surrounding ether undulations 

 which speed through it with the velocity of light. Our 



