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 its way through every capillary till it reaches another blood-vessel that carries it back to the heart.

It leaves the heart charged with carbonic acid and watery vapor.