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RADIATION. 1 95

lavender 60, that of rosemary 74, while that of aniseseed
amounts to 372. It would be idle to speculate on the
quantities of matter concerned in these actions.

12. Aqueous Vapor in relation to the Terrestrial
Temperatures.‘

We are now fully prepared for a result which, without
such preparation, might appear incredible. Water is, to
some extent, a volatile body, and our atmosphere, resting
as it does upon the surface of the ocean, receives from it a
continual supply of aqueous vapor. It would be an error
to confound clouds or fog or any visible mist with the va-
por of water: this vapor is a perfectly impalpable gas, dif-
fused, even onthe clearest days, throughout the atmosphere.
Compared with the great body of the air, the aqueous vapor
it contains is of almost infinitesimal amount, 99% out of
every 100 parts of the atmosphere being composed of oxy-
gen and nitrogen. In the absence of experiment, we should
never think of ascribing to this scant and varying conStitu-
ent any important influence on terrestrial radiation; and
yet its influence is far more potent than that of the great
body of the air. To say that on a day of average humidity
in England, the atmospheric vapor exerts 100 times the
action of the air itself, would certainly be an understate-
ment of the fact. The peculiar qualities of this vapor, and
the circumstance that at ordinary temperatures it is very
near its point of condensation, render the results which it
yields in the apparatus already described, less than the
truth; and I am not prepared to say that the absorption by
this substance is not 200 times that of the air in which it is
diffused. Comparing a single molecule of aqueous vapor
with an atom of either of the main constituents of our at-
mosphere, I am not prepared to say how many thousand
times the action of the former exceeds that of the latter.

1 See Note at the end of this Lecture.

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