hold as part of
our daily life, without which we can not live, and which we yet
contentedly poison nine times out of ten?

Oxygen, nitrogen, carbonic acid, and watery vapor; the last two being a
small portion of the bulk, oxygen and nitrogen making up four-fifths.
Small as the proportion of oxygen seems, an increase of but one-fifth more
would be destruction. It is the life-giver, but undiluted would be the
life-destroyer; and the three-fifths of nitrogen act as its diluent. No
other element possesses the same power. Fires and light-giving combustion
could not exist an instant without oxygen. Its office seems that of
universal destruction. By its action decay begins in meat or vegetables
and fruits; and it is for this reason, that, to preserve them, all oxygen
must be driven out by bringing them to the boiling point, and sealing them
up in jars to which no air can find entrance. With only undiluted oxygen
to breathe, the tissues would dry and shrivel, fuel burn with a fury none
could withstand, and every operation of nature be conducted with such
energy as soon to exhaust and destroy all power. But "a mixture of the
fiery oxygen and inert nitrogen gives us the golden mean. The oxygen now
quietly burns the fuel in our stoves, and keeps us warm; combines with the
oil in our lamps, and gives us light; corrodes our bodies, and gives us
strength; cleanses the air, and keeps it fresh and invigorating; sweetens
foul water, and makes it wholesome; works all around us and within us a
constant miracle, yet with such delicacy and quietness, we never perceive
or think of it, until we see it with the eye of science."

Food and air are the two means by which bodies live. In the full-grown
man, whose weight will average about one hundred and fifty-four pounds,
one hundred and eleven pounds is oxygen drawn from the air we breathe.
Only when food has been dissolved in the stomach, absorbed at last into
the blood, and by means of circulation brought into contact with the
oxygen of the air taken into our lungs, can it begin to really feed and
nourish the body; so that the lungs may, after all, be regarded as the
true stomach, the other being not much more than the food-receptacle.

Take these lungs, made up within of branching tubes, these in turn formed
by myriads of air-cells, and each air-cell owning its network of minute
cells called _capillaries_. To every air-cell is given a blood-vessel
bringing blood from the heart, which finds i