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. It returns,
if pure air has met it in the lung, with all corruption destroyed, a
dancing particle of life. But to be life, and not slow death, thirty-three
hogsheads of air must pass daily into the lungs, and twenty-eight pounds
of blood journey from heart to lungs and back again three times in each
hour. It rests wholly with ourselves, whether this wonderful tide, ebbing
and flowing with every breath, shall exchange its poisonous and clogging
carbonic acid and watery vapor for life-giving oxygen, or retain it to
weigh down and debilitate every nerve in the body.

With every thought and feeling some actual particles of brain and nerve
are dissolved, and sent floating on this crimson current. With every