To prevent misunderstanding,
nevertheless, it will be useful to explain the controversy about
the climate of the earth in past ages which divides modern
geologists.
The root of the difference of opinion and the character of the
conflicting parties have already been indicated. It is a protest
of the "Planetesimalists" against the older, and still general,
view of the origin of the earth. As we saw, that view implies
that, as the heavier elements penetrated centreward in the
condensing nebula, the gases were left as a surrounding shell of
atmosphere. It was a mixed mass of gases, chiefly oxygen,
hydrogen, nitrogen, and carbon-dioxide (popularly known as
"carbonic acid gas"). When the water-vapour settled as ocean on
the crust, the atmosphere remained a very dense mixture of
oxygen, nitrogen, and carbon-dioxide--to neglect the minor gases.
This heavy proportion of carbon-dioxide would cause the
atmosphere to act as a glass-house over the surface of the earth,
as it does still to some extent. Experiment has shown that an
atmosphere containing much vapour and carbon-dioxide lets the
heat-rays pass through when they are accompanied by strong light,
but checks them when they are separated from the light.
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