A good experiment to illustrate
the tendency of the warm impure
air of a room that is occupied to
escape may be made in the follow-
ing manner. Let a door leading
from a warm room to one unoccu-
pied, or to a hall be partly opened
and a lighted candle held at the
opening near to the top, and then
near the bottom; when near the
top the flame will be seen to trail
outward by the current of warm
air that is escaping, but when held
near the bottom the flame will be
bent inward by the entrance of
colder air to supply the place of
that which is escaping.
242. There are some phenomena
attending the passage of air
through tubes, and its escape from
them in certain ciicumstances,
which are not a little curious. If a tube be made of
tissue-paper, 6 or 8 inches long, and about an inch or
a little less in diameter, having a piece of wood in
one end with a hole in its centre a quarter of an inch
in diameter, on blowing through this hole, either directly
or by means of a small tube, the paper will collapse,
plainly indicating the production of a partial vacuum
within it. This we may suppose to be occasioned by the
sudden expansion of the air on escaping from the small
tube by which it was introduced within the paper tube.
A portion of the air within the paper is blown away, and
the tendency of the air outside to rush in and supply the
vacuum, produces the collapse we have noticed.
