The branch of the physical sciences which relates to the air and its
various phenomena is called Pneumatics. By it we learn many curious
particulars. By it we find that the air has weight and pressure,
color, density, elasticity, compressibility, and some other properties
with which we shall endeavor to make the young reader acquainted, by
many pleasing experiments, earnestly impressing upon him to lose no
opportunity of making physical science his study.
To show that the air has weight and pressure, the common leather sucker
by which boys raise stones will show the pressure of the atmosphere. It
consists of a piece of soft but firm leather, having a piece of string
drawn through its center. The leather is made quite wet and pliable,
and then its under part is placed upon the stone and stamped down by
the foot. This pressing of the leather excludes the air from between
the leather and the stone, and by pulling the string a vacuum is left
underneath its center; consequently the weight of the air about the
edges of the leather not being counterbalanced by any air between it
and the stone, enables the boy to lift it.
The Magic Tumbler.
The air which for about forty miles surrounds our earth has a definite
weight; and although we can neither see nor feel it, we are conscious
of its presence by the momentary operation of breathing. The weight of
a column of air one inch square, and forty miles high, is about fifteen
pounds.
The reason why we are not crushed down by this enormous weight is
because we are surrounded on all sides by it, and as the pressure of
weight is equal all around, it becomes, as far as we are personally
concerned, insensible.
That the air _does_ exert a definite pressure, in consequence of
its weight, may be easily proved by any one with the above simple
apparatus--only a tumbler and a sheet of paper. Fill a tumbler quite
full of water, and carefully draw over its top a sheet of clean letter
paper, and be careful to see that there are no bubbles of air in the
water; place your hand over the paper while inverting it, and when
the glass is mouth downward the water will be kept in, until the paper
becomes wet through. The air pressing against the mouth of the tumbler
is of greater weight than the contained water, and so, until some air
can get in to supply the place of the water, it cannot fall out.
The Weight of the Air Proved by a Pair of Bellows.
Shut the nozzle and valve-hole of a pair of bellows, and after having
squeezed the air out of them, if they are perfectly air-tight, we
shall find that a very great force, even some hundreds of pounds, is
necessary for separating the boards. They are kept together by the
weight of the heavy air which surrounds them, in the same manner as if
they were surrounded by water.
The Revolving Serpent.
This illustration represents an amusing and instructive experiment,
which proves the ascension of heated air by rendering its effects
visible, and it may also be used to test the direction of the currents
in our rooms and dwellings. To construct one, a piece of card-board
is taken and cut in the form of a spiral, and to give effect it may
be painted to represent a serpent. Then prepare a stand, having a
needle in its upper end, and suspend the serpent from its center on the
needle. If this be now placed over a stove, or the tail of the serpent
suspended by a bit of thread over a lamp, the heated air ascending
through it will cause it to revolve in a very amusing manner. Two
serpents may be made to turn in opposite directions, by pulling out
one from the one side, and the other in the reverse direction, so that
their heads may point toward each other when suspended.
To Put a Lighted Candle Under Water.
Procure a good-sized cork, or bung; upon this place a small, lighted
taper; then set it afloat in a pail of water. Now, with a steady hand,
invert a large drinking glass over the light, and push it carefully
down into the water. The glass being full of air, prevents the water
from entering it. You may thus see the candle burn _under_ water,
and bring it up again to the surface, still alight. This experiment,
simple as it is, serves to elucidate that useful contrivance called the
diving-bell, being performed on the same principle.
The largest drinking-glass holds but half a pint, so that your diving
light soon goes out for the want of air. As an average, a burning
candle consumes as much air as a man, and he requires nearly a gallon
of air every minute, so that, according to the size of the glass over
the flame, you can calculate how many seconds it will remain alight; of
course, a large flame requires more air than a small one. For this, and
several other experiments, a quart bell-glass is very useful, but being
expensive it is not found in every parlor laboratory: one is, however,
easily made from a green glass pickle-bottle; get a glazier to cut off
the bottom, and you have a bell-glass that Chilton would not reject.
To Place Water in a Drinking-Glass Upside Down.
Procure a plate, a tumbler, and a small piece of tissue or silver
paper. Set the plate on a table, and pour water in it up to the first
rim. Now slightly crumple up the paper, and place it in the glass;
then set it on fire. When it is burnt out, or rather just as the last
flame disappears, turn the glass quickly upside down into the water.
Astonishing! the water rushes with great violence into the glass! Now
you are satisfied that water can be placed in a drinking-glass upside
down. Hold the glass firm, and the plate also. You can now reverse the
position of the plate and glass, and thus convince the most skeptical
of the truth of your pneumatic experiment. Instead of burning paper,
a little brandy or spirits of wine can be ignited in the glass; the
result of its combustion being invisible, the experiment is cleaner.