of pure air to every inmate, will now be described and illustrated.

It is the common property of both air and water to expand, become
lighter and rise, just in proportion as they are heated; and therefore
it is the invariable law that cool air sinks, thus replacing the warmer
air below. Thus, whenever cool air enters a warm room, it sinks downward
and takes the place of an equal amount of the warmer air, which is
constantly tending upward and outward. This principle of all fluids
is illustrated by the following experiment:

Take a glass jar about a foot high and three inches in diameter, and
with a wire to aid in placing it aright, sink a small bit of lighted
candle so as to stand in the centre at the bottom. (Fig. 28.) The
candle will heat the air of the jar, which will rise a little on one
side, while the colder air without will begin falling on the other
side. These two currents will so conflict as finally to cease, and
then the candle, having no supply of oxygen from fresh air, will begin
to go out. Insert a bit of stiff paper so as to divide the mouth of
the jar, and instantly the cold and warm air are not in conflict as
before, because a current is formed each side of the paper; the cold
air descending on one aide and the warm air ascending the other side,
as indicated by the arrows. As long as the paper remains, the candle
will burn, and as soon as it is removed, it will begin to go out, and
can be restored by again inserting the paper.

[Illustration: Fig. 28]
[Illustration: Fig. 29]

This illustrates the mode by which coal-mines are ventilated when
filled with carbonic acid. A shaft divided into two passages, (Fig.