A DROP OF WATER. 85 
up into food for the plant, and only if the leaf has 
more water than it needs, some drops may escape at 
the tiny openings under the leaf, and be drawn up 
again by the sun-waves as invisible vapour into the air. 
Again, much of the rain falls on hard rock and 
stone, where it cannot sink in, and then it lies in pools 
till it is shaken apart again into vapour and carried off 
in the air. Nor is it idle here, even before it is car- 
ried up to make clouds. We have to thank this in- 
visible vapour in the air for protecting us from the 
burning heat of the sun by day and intolerable frost 
by night. 
Let us for a moment imagine that we can see all 
that we know exists between us and the sun. First, 
we have the fine ether across which the sunbeams 
travel, beating down upon our earth with immense 
force, so that in the sandy desert they are like a burn- 
ing fire. Then we have the coarser atmosphere of oxy- 
gen and nitrogen atoms hanging in this ether, and 
bending the minute sun-waves out of their direct path. 
But they do very little to hinder them on their way, 
and this is why in very dry countries the sun's heat is 
so intense. The rays beat down mercilessly, and noth- 
ing opposes them. Lastly, in damp countries we have 
the larger but still invisible particles of vapour hang- 
ing about among the air-atoms. Now, these watery 
particles, although they are very few (only about one 
twenty-fifth part of the whole atmosphere), do hinder 
the sun-waves. For they are very greedy of heat, and 
though the light-waves pass easily through them, they 
catch the heat-waves and use them to help themselves 
to expand. And so, when there is invisible vapour in 
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