A. Because the carbonic acid gas escapes through the vent hole.
Q. Why will not beer run out of the tub, till the vent peg is taken out?
A. When the tap is turned, air rushes through the tap into the bottom of the tub, and holds the liquor in.
The upward pressure of air is illustrated by the
following simple experiment:—Fill a wine-glass with water; cover the top of the glass with a piece of writing paper; turn the glass topsy turvy, and the water will not run out. The paper is used merely to give the air a medium sufficiently dense to act against.
Q. Why does the beer run freely, immediately the vent peg is taken out?
A. As soon as the vent peg is taken out, air rushes through the vent hole at the top of the tub,—presses the liquor down, and forces it through the tap.
Q. Why does liquor flow reluctantly out of a bottle held upside down?
A. Because the upward pressure of the air prevents the liquor from flowing out.