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The second most important constituent of the biosphere is 3V`K^X3
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liquid water. This can only exist in a very narrow range of temperatures, &a8#q v"l
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since water freezes at 0°C and boils at 100°C. This is %H- [u}s
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only a tiny range compared with the low temperatures of some ##u+[ !
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other planets and the hot interior of the earth, let the temperature __1__ CBSJY&:K
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of the sun. by controlpanel `+B+RQl}[
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As we know, life would only be possible on the face of a __2__ wXZ . D}d
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planet had temperatures somewhere within this range. __3__ [X^JV/R
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The earth’s supply of water probably remains quite fairly constant __4__ (fh:q 2E#
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in quantity. A certain number of hydrogen atoms, which &O0+\A9tP
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are one of the main constituents of water, are lost by escaping q3NS?t!
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from the atmosphere to out space, but they are probably just __5__ ;S?1E:\av
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about to be replaced by new water rising away from the depths of the __6__ Fa+PN9M`?.
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earth during volcanic action. The total quantity of water is not ziH2<@
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known, and it is about enough to cover the surface of the globe __7__ ['s_qCA [
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to a depth of about two and three-quarter kms. Most of it—97% lC _zSmT
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is in the form of the salt waters of the oceans. The rest is e
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fresh, but three quarter of this is in the form of ice at the Poles __8__ }SN'*w@E
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and on mountains, and cannot be used by living systems when __9__ r?X^*o9
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melted. Of the remaining fraction, which is somewhat fewer __10__ F_r eBPx
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than 1%of the whole, there is 10-20 times as much stored as underground yv:NH|,/y
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water as is actually on the surface. There is also a minor,
