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The second most important constituent of the biosphere is liquid water. This can only exist in a very narrow range of temperatures,since water freezes at 0°C and boils at 100°C. This is only a tiny range compared with the low temperatures of some other planets and 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.
