e blood through the capillaries, and the
absorption of carbon dioxide from the tissues.

The _lungs_ are two of the great purifiers of the blood. As it
circulates through them, it gives up the carbon dioxide which it has
absorbed, and receives pure oxygen in exchange. If the air of a room is
"foul," the blood does not get the proper amount of oxygen. For this
reason it is advisable for us to keep the windows of our rooms open as
much as possible both day and night. Fatigue is caused by the
accumulation of carbon dioxide and other impurities in the blood. When
we run, the heart pumps blood through the lungs faster than they can
purify it, and eventually our muscles become poisoned to such an extent
that we have to stop from sheer exhaustion.

THE COURSE OF THE BLOOD.

It takes rather less than a minute for a drop of blood to circulate from
the heart through the whole system and back to the heart.

We may briefly summarize the course of the circulation of the blood
thus:--It is expelled from the left ventricle into the _aorta_ and the
main arteries, whence it passes into the smaller arteries, and thence
into the capillaries of the brain, stomach, kidneys, etc. It here
imparts oxygen to the body, and takes in impurities. It then enters the
veins, and through them flows back to the right auricle; is driven into
the right ventricle; is expelled into the _pulmonary_ (lung)
_arteries_; enters the lungs, and is purified. It returns to the left
auricle through the _pulmonary veins_; enters the left auricle, passes
to left ventricle, and so on.

A healthy heart beats from 120 times per minute in a one-year-old infant
to 60 per minute in a very aged person. The normal rate for a
middle-aged adult is from 80 to 70 beats.

Heart disease signifies the failure of the heart valves to close
properly. Blood passes back when the heart contracts, and the
circulation is much enfeebled. By listening through a stethoscope the
doctor is able to tell whether the valves are in good order. A hissing
sound during the beat indicates a leakage past the valves; a thump, or
"clack," that they shut completely.

THE HYDRAULIC PRESS.

It is a characteristic of fluids and gases that if pressure be brought
to bear on any part of a mass of either class of bodies it is
transmitted equally and undiminished in all directions, and acts with
the same force on all equal surfaces, at right angles to those surfaces.
The great natural philosopher Pasca