[16]
[Footnote 16: The discovery of free hydrogen in the earth's atmosphere,
by Professor Dewar, 1901, bears upon the theory of the escape of gases
from a planet, and may modify the view above expressed. Since hydrogen
is theoretically incapable of being permanently retained in the free
state by the earth, its presence in the atmosphere indicates either that
there is an influx from space or that it emanates from the earth's
crust. In a similar way it may be assumed that atmospheric gases can be
given off from the crust of the moon, thus, to a greater or less extent,
supplying the place of the molecules that escape.]
After it had been found that, to ordinary tests, the moon offered no
evidence of the possession of an atmosphere, and before Dr. Stoney's
theory was broached, it was supposed by many that the moon had lost its
original supply of air by absorption into its interior. The oxygen was
supposed to have entered into combination with the cooling rocks and
minerals, thus being withdrawn from the atmosphere, and the nitrogen was
imagined to have disappeared also within the lunar crust. For it seems
to have always been tacitly assumed that the phenomenon to be accounted
for was not so much the _absence_ of a lunar atmosphere as its
_disappearance_. But disappearance, of course, implies previous
existence. In like manner it has always been a commonly accepted view
that the moon probably once had enough water to form lakes and seas.
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