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RADIATION. l 9 9

change their relative powers of absorption. N othng 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 thiswonderful 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. Reciprootiy 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

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