Lastly, there is a great deal of water in the air, floating about
as invisible vapour or water-dust, and this we shall speak of in
the next lecture. Still, all these gases and vapours in the
atmosphere are in very small quantities, and the bulk of the air
is composed of oxygen and nitrogen.

Having now learned what air is, the next question which presents
itself is, Why does it stay round our earth? You will remember
we saw in the first lecture, that all the little atoms of a gas
are trying to fly away from each other, so that if I turn on this
gas-jet the atoms soon leave it, and reach you at the farther end
of the room, and you can smell the gas. 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