Perhaps you will say, If oxygen is so useful, why is not the air
made entirely of it? But think for a moment. If there was such
an immense quantity of oxygen, how fearfully fast everything
would burn! Our bodies would soon rise above fever heat from the
quantity of oxygen we should take in, and all fires and lights
would burn furiously. In fact, a flame once lighted would spread
so rapidly that no power on earth could stop it, and everything
would be destroyed. So the lazy nitrogen is very useful in
keeping the oxygen-atoms apart; and we have time, even when a
fire is very large and powerful, to put it out before it has
drawn in more and more oxygen from the surrounding air. Often,
if you can shut a fire into a closed space, as in a closely-shut
room or the hold of a ship, it will go out, because it has used
up all the oxygen in the air.

So, you see, we shall be right in picturing this invisible air
all around us as a mixture of two gases. But when we examine
ordinary air very carefully, we find small quantities of other
gases in it, besides oxygen and nitrogen. First, there is
carbonic acid gas. This is the bad gas which we give out of our
mouths after we have burnt up the oxygen with the carbon of our
bodies inside our lungs; and this carbonic acid is also given out
from everything that burns. If only animals lived in the world,
this gas would soon poison the air; but plants get hold of it,
and in the sunshine they break it up again, as we shall see in
Lecture VII, and use up the carbon, throwing the oxygen back into
the air for us to use. Secondly, there are very small quantities
of ammonia, or the gas which almost chokes you in smelling-salts,
and which, when liquid is commonly called "spirits of hartshorn."
This ammonia is useful to plants, as we shall see by and by.