Why, then, do not all
the atoms of oxygen and nitrogen fly away from our earth into
space, and leave us without any air?
Ah! here you must look for another of our invisible forces.
Have you forgotten our giant force, "gravitation," which draws
things together from a distance? This force draws together the
earth and the atoms of oxygen and nitrogen; and as the earth is
very big and heavy, and the atoms of air are light and easily
moved, they are drawn down to the earth and held there by
gravitation. But for all that, the atmosphere does not leave off
trying to fly away; it is always pressing upwards and outwards
with all its might, while the earth is doing its best to hold it
down.
The effect of this is, that near the earth, where the pull
downward is very strong, the air-atoms are drawn very closely
together, because gravitation gets the best of the struggle. But
as we get farther and farther from the earth, the pull downward
becomes weaker, and then the air-atoms spring farther apart, and
the air becomes thinner.
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