GASES AND LIQUIDS—PRESSURE OF THE AIR—EXPERIMENTS.
We have more than once referred to the pressure of the air which exerts
a great influence upon bodies in motion, but a few experiments will
make this more obvious, and clearly demonstrate the fact. We have also
told you some of the properties of Solids, such as Weight, Inertia,
Friction, and Resistance, or Strength. Solids also, as we have seen,
occupy space, and cannot be readily compressed, nor bent to other
shapes. Now the subject of the Pressure of the Air leads us to the
other forms of Matter; namely, Gases and Liquids, which will be found
very interesting to study.
[Illustration: Fig. 38.—Blowing an egg from one glass to another.]
The force of air can very soon be shown as acting with considerable
pressure upon an egg in a glass. By blowing in a claret glass
containing a hard-boiled egg, it is possible to cause the egg to
jump out of the glass; and with practice and strength of lungs it
is not impossible to make it pass from one glass to another, as per
illustration (fig. 38).
The force of heated air ascending can also be ascertained by cutting up
a card into a spiral, and holding it above the flame of a lamp (fig.
39). The spiral, if lightly poised, will turn round rapidly.
Now let us turn to a few experiments with the air, which is composed
in two gases, Oxygen and Nitrogen, of which we shall hear more when we
come to CHEMISTRY.
[Illustration: Fig. 39.—Movement of heated air.]
[Illustration: Fig. 40.—Pressure of the air.]
It is not intended here to prosecute researches, but rather to sketch
a programme for instruction, based on amusing experiments in Physics,
performed without apparatus. The greater part of these experiments
are probably well known, and we desire to say that we merely claim to
have collected and arranged them for our descriptions. We must also
add that we have performed and verified these experiments; the reader,
therefore, can attempt them with every certainty of success. We will
suppose that we are addressing a young auditory, and commence our
course of Physics with some facts relating to the pressure of air. A
wine glass, a plate, and water, will serve for our first experiments.
Pour some water on the plate, light a piece of paper resting on a cork,
and cover the flame with the glass which I turn upside down (fig. 40).
What follows?—The water rises in the glass. Why?—Because the burning
of the paper having absorbed a part of the oxygen, and the volume
of confined gas being diminished, the pressure of the outer air has
driven back the fluid. I next fill a goblet with water up to the brim,
and cover it with a sheet of paper which touches both the edge of the
glass and the surface of the water. I turn the glass upside down, and
the sheet of paper prevents the water running out, because it is held
in place by atmospheric pressure (fig. 41). It sometimes happens that
this experiment does not succeed till after a few attempts on the part
of the operator; thus it is prudent to turn the glass over a basin, so
that, in case of failure, the water is not spilt. Having obtained a
vase and a bottle, both quite full of water, take the bottle, holding
it round the neck so that the thumb can be used as a stopper, then turn
it upside down, and pass the neck into the water in the vase. Remove
your thumb, or stopper, keeping the bottle in a vertical position, and
you will see that the water it contains does not escape, but remains in
suspension. It is atmospheric pressure which produces this phenomenon.
If, instead of water, we put milk in the bottle, or some other fluid
denser than water, we shall see that the milk also remains suspended
in the bottle, only there is a movement of the fluid in the neck of
the bottle, and on careful examination we perceive very plainly that
the milk descends to the bottom of the vase, and the water rises into
the bottle. Here, again, it is atmospheric pressure which maintains
the fluid in the bottle, but the milk descends, because fluids are
superposed according to their order of density, and the densest liquid
falls to the bottom.
This can be verified by means of the _phial of the four elements_,
which is a plain, long, and narrow bottle, containing equal volumes of
metallic mercury, salt water, alcohol, and oil. These four liquids will
lie one on the top of the other without ever mixing, even if shaken.
Another experiment as to the pressure of the air may be made (fig. 42).
Take a penny and press it against some oaken bookcase or press, rub the
coin against the wood for a few seconds, then press it, and withdraw
the fingers. The coin will continue to adhere to the wood. The reason
of this is, because by the rubbing and the pressure you have dispersed
the film of air which was between the penny and the wood, and under
those conditions the pressure of the atmospheric air was sufficient to
keep the penny in its place.
[Illustration: Fig. 41.—Pressure of the air.]
Or, again, let us now add a water-bottle and a hard-boiled egg to
our appliances; we will make use of the air-pump, and easily perform
another experiment. I light a piece of paper, and let it burn, plunging
it into a water-bottle full of air. When the paper has been burning
a few seconds I close the opening of the water-bottle by means of a
hard-boiled egg, which I have previously divested of its shell, so that
it forms a hermetic stopper. The burning of the paper has now caused a
vacuum of air in the bottle, and the egg is gradually thrust in by the
atmospheric pressure outside. Fig. 43 exhibits it slowly lengthening
and stretching out as it passes through the aperture; then it is
suddenly thrust completely into the bottle with a little explosive
sound, like that produced by striking a paper bag expanded with air.
This is atmospheric pressure demonstrated in the clearest manner, and
at little cost.
[Illustration: Fig. 42.—Coin adhering by pressure of air.]
If it is desired to pursue a little further the experiments relating
to atmospheric pressure, it will be easy enough to add to the
before-mentioned appliances a closed glass-tube and some mercury, and
one will then have the necessary elements for performing Torricelli’s
and Pascal’s experiments, and explaining the theory of the barometer
(page 52).
An amusing toy, well-known to schoolboys, called the “sucker,” may also
be made the object of many dissertations on the vacuum and the pressure
of air. It is composed of a round piece of soft leather, to the centre
of which is attached a small cord. This leather is placed on the
ground and pressed under foot, and when the cord is pulled it forms a
cupping-glass, and is only separated with difficulty from the pavement.
Atmospheric air, in common with other gases, has a tendency to fill
any space into which it may enter. The mutual attraction of particles
of air is _nil_; on the contrary, they appear to have a tendency to
fly away from each other; this property is called “repulsion.” Air
also possesses an expansive property—a tendency to press against all
the sides of any vessel in which it may be enclosed. Of course the
larger the vessel containing a given quantity of air, the less actual
pressure it will exert on the sides of the vessel. The elasticity of
air therefore decreases with increasing expansion, but it gains in
elasticity or force when compressed.
There is a law in Physics which expresses the relation between
expansion and elasticity of gases, which may be said to be as follows:—
The elasticity (of a gas) is in inverse ratio to the space it occupies,
and therefore by compressing air into a small space we can obtain a
great force, as in the air-gun and the pop-gun of our youthful days.
[Illustration: Fig. 43.—Hard boiled egg, divested of its shell, passing
through the neck of a glass bottle, under the influence of atmospheric
pressure.]
In the cut below we can illustrate the principle of the pop-gun. The
chamber full of air is closed by a cork and by an air-tight piston
(S) at _p_ and _p_. When the piston is pushed into the chamber
the air is compressed between it and the stopper, which at length flies
out forcibly with a loud report
[Illustration: Fig. 44.—The principle of the pop-gun.]
We have said that the tendency of air particles is to fly away from
each other, and were it not for the earth’s attraction the air might be
dispersed. The height of the atmosphere has been variously estimated
from a height of 45 miles to 212 miles in an attenuated form; but
perhaps 100 miles high would be a fair estimate of the height to which
our atmosphere extends.
[Illustration: Fig. 45.—Weighing the air.]
The pressure of such an enormous body of gas is very great. It has
been estimated that this pressure on the average human body amounts
to fourteen tons, but being balanced by elastic fluids in the body,
the inconvenience is not felt. The _Weight_ of Air can easily be
ascertained, though till the middle of the seventeenth century the
air was believed to be without weight. The accompanying illustration
will prove the weight of air. Take an ordinary balance; and suspend
to one side a glass globe fitted with a stop-cock. From this globe
extract the air by means of the air-pump, and weigh it. When the exact
weight is ascertained turn the stop-cock, the air will rush in, and the
globe will then pull down the balance, thus proving that air possesses
weight. The experiments of Torricelli and Otto von Guerike, however,
demonstrated that the air has weight and great pressure. Torricelli
practically invented the barometer, but Otto von Guerike, by the cups
known as _Magdeburg Hemispheres_, proved the pressure of the outward
air. This apparatus is well known, and consists of two hollow copper
hemispheres which fit very closely. By means of the air-pump which he
invented in 1650, Otto von Guerike exhausted the air from the closed
hemispheres. So long as air remained in them, there was no great
difficulty in separating them; but when it had been finally exhausted,
the pressure of the surrounding atmosphere was so great that the hollow
spheres could not be dragged asunder even by horses harnessed to rings
which had been inserted in the globes.
[Illustration: Fig. 46.—Magdeburg Hemispheres.]
The _Air-Pump_ is a very useful machine, and we will now briefly
explain its action. The inventor was, as remarked above, Otto von
Guerike, of Magdeburg. The pump consists of a cylinder and piston and
rod, with two valves opening upwards—one valve being in the bottom of
the cylinder, the other in the piston. This pump is attached by a tube
to a plate with a hole in it, one extremity of the tube being fixed in
the centre of the plate, and the other at the valve at the bottom of
the cylinder. A glass shade, called the _receiver_, is placed on the
top of the plate, and of course this shade will be full of air (fig.
47).
[Illustration: Fig. 47.—The air-pump.]
When the receiver is in position, we begin to work the pump. We have
said there are two valves. So when the piston is drawn up, the cylinder
would be quite empty did not the valve at the bottom, opening upwards,
admit some air from the glass shade through the tube to enter the
cylinder. Now the lower part of the cylinder is full of air drawn from
the glass shade. When we press the piston down again, we press against
the air in it, which, being compressed, tries to escape. It cannot go
back, because the valve at the bottom of the cylinder won’t open, so
it escapes by the valve in the piston, and goes away. Thus a certain
amount of air is got rid of at each stroke of the piston. Two cylinders
and pistons can be used, and so by means of cog-wheels, etc., the air
may be rapidly exhausted from the receiver. Many experiments are made
with the assistance of the air-pump and receiver, though the air is
never _entirely_ exhausted from the glass.
The “Sprengel” air-pump is used to create an almost perfect vacuum,
by putting a vessel to be exhausted in connection with the vacuum at
the top of a tube of mercury thirty inches high. Some air will bubble
out, and the mercury will fall. By filling up again and repeating the
process, the air vessel will in time be completely exhausted. This
is done by Mr. Sprengel’s pump, and a practically perfect vacuum is
obtained, like the _Torricellian_ vacuum.
The “Torricellian vacuum” is the empty space above the column of
mercury in the barometer which we will proceed to describe. Air has a
certain weight or pressure which is sufficient to raise a column of
mercury thirty inches. We will prove this by illustration. Take a bent
tube and fill it with mercury; the liquid will stand equally high in
both arms, in consequence of the ratio of equilibrium in fluids, of
which we shall read more when we come to consider Water. So the two
columns of mercury are in equilibrium. (See A.) Now stop the
arm a with a cork, and take out half the mercury. It will remain in one
arm only. Remove the cork, and the fluid will fall in both arms, and
remain in equilibrio. If a long bent glass tube be used, the arms being
thirty-six inches high, the mercury will fall to a point _c_, which
measures 29·9 inches from the bottom. If the tube be a square inch
in bore, we have 29·9 cubic inches of mercury, weighing 14⅘ lbs.,
balancing a column of air one square inch thick and as high as the
atmosphere. So the mercury and the column of air must weigh the same.
Thus every square inch on the earth supports a weight of (nearly) 15
lbs (figs. 48 and 50).
[Illustration: Fig. 48.—Air Pressure.]
[Illustration: Fig. 49.—The Barometer.]
The barometer invented by Pascal, working on the investigations of
Torricelli, is a very simple and useful instrument. Fill a tube with
mercury from which all moisture has been expelled, and turn it over
in a dish of mercury; the mercury will rise to a certain height (30
inches), and no higher in vacuo. When the pressure of the air increases
the mercury rises a little, and falls when the pressure is removed. Air
charged with aqueous vapour is lighter than dry air, so a fall in the
mercury indicates a certain amount of water-vapour in the air, which
may condense and become rain. The action of mercury is therefore used
as a weather-glass, by which an index-point shows the movements of the
fluid, by means of a wheel over which a thread passes, sustaining a
float and a counterpoise. When the mercury rises the float goes up,
and the weight falls, and turns the wheel by means of the thread. The
wheel having a pointer on the dial tells us how the mercury moves. This
_weather-glass_ is the usual _syphon barometer_ with the float on the
surface and a weight (fig. 50).
[Illustration: Fig. 50.—Syphon barometer.]
The Syphon Barometer is a bent tube like the one already shown, with
one limb much shorter than the other.
The Aneroid Barometer, so called because it is “without moisture,”
is now in common use. In these instruments the atmospheric pressure
is held in equilibrium by an elastic metal spring or tube. A metal
box, or tube, is freed from air, and then hermetically sealed. This
box has a flexible side, the elasticity of which, and the pressure of
the air on it, keep each other in equilibrium. Upon this elastic side
the short arm of a lever is pressed, while the longer arm works an
index-point, as in the circular barometer. When pressure increases the
elastic box “gives”; when pressure diminishes it returns to its former
place, and the index moves in the opposite direction. It is necessary
to compare and “set” the aneroid with the mercurial barometer to ensure
correctness. A curved tube is sometimes used, which coils and uncoils
like a spring, according to the pressure on it.
[Illustration: Fig. 51.—The Water Barometer.]
There are other barometers, such as the Water Barometer, which can
be fixed against the side of a house, and if the water be coloured,
it will prove a useful indicator. As the name indicates, water is
used instead of mercury, but as the latter is thirteen-and-half times
heavier than water, a much longer tube is necessary; viz., one about
thirty-five feet in length. The construction is easy enough. A leaden
pipe can be fixed against the house; on the top is a funnel furnished
with a stop-cock, and placed in a vase of water. The lower part of the
tube is bent, and a glass cylinder attached, with another stop-cock—the
glass being about three feet long, and graduated. Fill the tube with
water, shut the upper stop-cock, and open the lower one. The vacuum
will be formed in the top of the tube, and the barometer will act on a
larger scale than the mercury.
The Glycerine Barometer, invented by Mr. Jordan, and in use at the
_Times_ office, registers as more than one inch movements which on the
mercurial thermometer are only one-tenth of an inch, and so are very
distinctly visible. The specific gravity of pure glycerine is less
than one-tenth that of mercury, so the mean height of the glycerine
column is twenty-seven feet at sea level. The glycerine has, however,
a tendency to absorb moisture from the air, but Mr. Jordan, by putting
some petroleum oil upon the glycerine, neutralized that tendency, and
the atmospheric pressure remains the same. A full description of this
instrument was given in the _Times_ of 25th October, 1880.
[Illustration: Fig. 52.—The principle of the diving-bell.]
The uses of the barometer are various. It is employed to calculate the
heights of mountains; for if a barometer at sea level stand at 30",
it will be lower on a mountain top, because the amount of air at an
elevation of ten thousand feet is less than at the level of the sea,
and consequently exercises less pressure, and the mercury descends.
[The pressure is on the bulb of mercury at the bottom, not on the
_top_, remember.]
The pressure of the air at the tops of mountains sometimes decreases
very much, and it is not sufficiently dense for perfect respiration, as
many people find. Some climbers suffer from bleeding at the nose, etc.,
at great altitudes. This is occasioned by the action of the heart,
which pumps with great force, and the outward pressure upon the little
veins being so much less than usual, they give way.
[Illustration: Fig. 53.—Diver under water.]
Many important instruments depend upon atmospheric pressure. The
most important of these is the pump, which will carry us to the
consideration of water and FLUIDS generally. The fire-engine
is another example, but we will now proceed to explain the diving-bell
already referred to.
Fig. 52 represents the experiment of the diving-bell, which is so
simple, and is explained below. It belongs to the same category of
experiments as those relating to the pressure of air and compression of
gas. Two or three flies have been introduced into the glass, and they
prove by their buzzing about that they are quite at their ease in the
rather confined space.
The DIVING-BELL in a crude form appears to have been used as
early as 1538. It was used by two Greeks in the presence of the Emperor
Charles V., and numerous spectators. In the year 1720 Doctor Halley
improved the diving-bell, which was a wooden box or chamber open at
the bottom. Air casks were used to keep the inmate supplied with air.
The modern diving-bell was used by Smeaton in 1788, and was made of
cast iron. It sinks by its own weight. The pressure of the air inside
is sufficient to keep the water out. Air being easily compressed, it
is always pumped in to keep the hollow iron “bell” full, and to supply
the workmen. There are inventions now in use by which the diver carries
a supply of air with him on his back, and by turning a tap can supply
himself for a long time at a distance from the place of descent,
and thus is able to dispense with the air-tube from the boat at the
surface. This apparatus was exhibited at the Crystal Palace some years
ago.
[Illustration: Fig. 54.—The Hand Fire-Engine.]
THE PUMP.
We have seen in the case of the Water Barometer that the pressure of
the air will sustain a column of water about thirty feet high. So the
distance between the lower valve and the reservoir or cistern must
not be more than thirty-two feet, practically the distance is about
twenty-five feet in pumps.
We can see by the illustration that the working is much the same as in
the air-pump. The suction pipe B is closed by the valve C, the cylinder
D and spout E are above, the piston rod F lifts the air-tight piston
in which is a valve H. When the piston is raised the valve C opens and
admits the water into the cylinder. When the piston is depressed the
valve C is closed, the water already in forces H open, and passing
through the piston, reaches the cylinder and the spout (fig. 55).
The hand fire-engine depends upon the action of compressed air, which
is so compressed by pumping water into the air chamber _a_. The tube is
closed at _g_, and the pumps _e e_ drive water into the air chamber.
At length the tap is opened, and the air drives the water out as it is
continually supplied (fig. 54).
Compressed air was also used for driving the boring machines in the
Mount Cenis tunnel. In this case also the air was compressed by water,
and then let loose, like steam, to drive a machine furnished with
boring instruments.
A pretty little toy may be made, and at the same time exemplify an
interesting fact in Physics. It is called the _ludion_, and it “lies in
a nut shell” in every sense. When the kernel has been extracted from
the shell, fasten the portions together with sealing wax, so that no
water can enter. At one end O, as in the illustration, leave
a small hole about as large as a pin’s head; fasten two threads to the
sealing wax, and to the threads a wooden doll. Let a weight be attached
to his waist. When the figure is in equilibrium, and will float, put it
into a jar of water, and tie a piece of bladder over the top. If this
covering be pressed with the finger, the doll will descend and remount
when the finger is removed. By quick successive pressure the figure may
be made to execute a _pas seul_. The reason of the movement is because
the slight cushion of air in the upper part of the vase is compressed,
and the little water thus caused to enter the nut shell makes it
heavier, and it descends with the figure (fig. 56).
[Illustration: Fig. 55.—The Pump.]
We have now seen that air is a gas, that it exercises pressure, that it
possesses weight. We know it can be applied to many useful purposes,
and that the air machines and inventions—such as the air-pump and
the “Pneumatic Despatch”—are in daily use in our laboratories, our
steam engines, our condensed milk manufactories, and in many other
industries, and for our social benefit. Compressed air is a powerful
motor for boring machinery in tunnels where steam cannot be used, even
if water could be supplied, for smoke or fire would suffocate the
workers. To air we owe our life and our happiness on earth.
Pneumatics, then, deals with the mechanical properties of elastic
fluids represented by air. A gas is an elastic fluid, and differs very
considerably, from water; for a gas will fill a large or small space
with equal convenience, like the genii which came out of the bottle and
obligingly retired into it again to please the fisherman. We have seen
that the pressure of the air is 14⅘ per square inch at a temperature
of 32°. It is not so easy to determine the pressure of air at various
times as that of water. We can always tell the pressure of a column of
water when we find the height of the column, as it is the weight of so
many cubic inches of the liquid. But the pressure of the atmosphere per
square inch at any point is equal to the weight of a vertical column
of air one inch square, reaching from that point to the limit of the
atmosphere above it. Still the density is not the same at all points,
so we have to calculate. The average pressure at sea level is 14·7
per square inch, and sustains a column of mercury 1 square inch in
thickness, 29·92, or say 30 inches high. These are the data upon which
the barometer is based, as we have seen.
[Illustration: Fig: 56.—The “Ludion.”]
In our article upon “Chemistry” we will speak more fully of the
atmosphere and of its constituents, etc.