the hot interior of the earth, let the temperature __1__
of the sun.
As we know, life would only be possible on the face of a __2__
planet had temperatures somewhere within this range. __3__
The earth’s supply of water probably remains quite fairly constant __4__
in quantity. A certain number of hydrogen atoms, which
are one of the main constituents of water, are lost by escaping
from the atmosphere to out space, but they are probably just __5__
about to be replaced by new water rising away from the depths of the __6__
earth during volcanic action. The total quantity of water is not
known, and it is about enough to cover the surface of the globe __7__
to a depth of about two and three-quarter kms. Most of it—97%
is in the form of the salt waters of the oceans. The rest is
fresh, but three quarter of this is in the form of ice at the Poles __8__
and on mountains, and cannot be used by living systems when __9__
melted. Of the remaining fraction, which is somewhat fewer __10__
than 1%of the whole, there is 10-20 times as much stored as underground
water as is actually on the surface. There is also a minor,
but extremely important, fraction of the water supply
which is present as water vapour in the atmosphere.
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