46 FRAGMENTS OF SCIENCE. 
the same. We have liberated the molecules from the 
bonds which trammel them more or less in 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 ivater, 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 our most powerful absorber of radiant heat. 
Its attenuation, however, diminishes its action. I have 
proved that a shell of air fcwo 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, Avhich 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 
