_Pneumatics treats of the mechanical properties and effects of air and
similar fluids_; these are called elastic fluids and gases, or aëriform
fluids.
_Hydro-pneumatics._ This is a compound word formed from two Greek words
signifying water and air; in its primary meaning it conveys the idea of
_the combined action of water and air or gas_.
[Illustration: FIG. 330.]
NOTE.—Fig. 330 is one of the simplest forms of an air pump. The
description accompanying Fig. 341 properly applies to this one.
_Air is the respirable fluid which surrounds the earth and forms its
atmosphere._ It is inodorous, invisible, insipid, colorless, elastic,
possessed of gravity, easily moved, rarefied and condensed, essential
to respiration and combustion, and is the medium of sound. It is
composed by volume of 20.7 parts of oxygen and 79.3 of nitrogen,
by weight, of 23 of oxygen and 77 of nitrogen. These gases are not
chemically united, but are mixed mechanically. Air contains also 1/2000
of carbon dioxide, some aqueous vapor, about one per cent. of argon,
and small varying amounts of ammonia, nitric acid, ozone, and organic
matter. The specific gravity of the air at 32° F is to that of water as
1 to 773, and 100 cubic inches of air at mean temperature and pressure
weighs 30-1/2 grains.
_Aëriform fluids are those which have the form of air._ Many of them
are invisible, or nearly so, and all of them perform very important
operations in the material world. But, notwithstanding that they
are in most instances imperceptible to our sight, _they are really
material, and possess all the essential properties of matter_. They
possess, also, in an eminent degree, all the properties which have
been ascribed to liquids in general, besides others by which they are
distinguished from liquids.
_Elastic fluids are divided into two classes, namely, 1, permanent
gases and, 2, vapors._ The gases cannot be easily converted into
the liquid state by any known process of art;* but the vapors are
readily reduced to the liquid form either by pressure or diminution of
temperature. There is, however, no essential difference between the
mechanical properties of both classes of fluids.
As the air which we breathe, and which surrounds us, is the most
familiar of all this class of bodies, it is generally selected as the
subject of Pneumatics. But it must be premised that the same laws,
properties and effects, which belong to air, belong in common, also, to
all aëriform fluids or gaseous bodies.
_There are two principal properties of air, namely, gravity and
elasticity._ These are called the principal properties of this class
of bodies, because they are the means by which their presence and
mechanical agency are especially exhibited.
Although the aëriform fluids all have weight, they appear to possess
_no cohesive attraction_.
The pressure of the atmosphere caused by its weight is exerted on
all substances, internally and externally, and it is a necessary
consequence of its fluidity. When the external pressure is artificially
removed from any part, it is immediately felt by the reaction of the
internal air.
Heat insinuates itself between the particles of bodies and forces them
asunder, in opposition to the attraction of cohesion and of gravity. It
therefore exerts its power against both the attraction of gravitation
and the attraction of cohesion. But, as the attraction of cohesion
does not exist in aëriform fluids, _the expansive power of heat upon
them has nothing to contend with but gravity_. Hence, any increase of
temperature, expands an elastic fluid prodigiously, and a diminution of
heat condenses it.
*NOTE.—Carbonic acid gas forms an exception to this assertion. Water
also is the union of oxygen and hydrogen gas.
A column of air, having a base an inch square, and reaching to the top
of the atmosphere, weighs about fifteen pounds. _This pressure, like
the pressure of liquids, is exerted equally in all directions._
_The elasticity of air_ and other aëriform fluids is that property by
which they are increased or diminished in extension, according as they
are compressed. This property exists in a much greater degree in air
and other similar fluids than in any other substance. In fact, it has
no known limit, for, when the pressure is removed from any portion of
air, it immediately expands to such a degree that the smallest quantity
will diffuse itself over an indefinitely large space. And, on the
contrary, when the pressure is increased, it will be compressed into
indefinitely small dimensions.
The elasticity or pressure of air and all gases is in direct
_proportion to their density_; or, what is the same thing, inversely
proportional to the space which the fluid occupies. This law, which
was discovered by Mariotte, is called “Mariotte’s Law.” This law may
perhaps be better expressed in the following language; namely, the
density of an elastic fluid is in direct proportion to the pressure
which it sustains.
_Air becomes a mechanical agent by means of its weight, its elasticity,
its inertia and its fluidity._
The fluidity of air invests it, as it invests all other liquids, with
the power of transmitting pressure; fluidity is a necessary consequence
of the independent gravitation of the particles of a fluid. It may,
therefore, be included among the effects of weight.
The inertia of air is exhibited in the resistance which it opposes to
motion, which has already been noticed under the head of Mechanics.
This is clearly seen in its effects upon falling bodies, as will be
exemplified in the experiments with the air-pump.
_The great degree of elasticity_ possessed by all aëriform fluids,
renders them susceptible of compression and expansion to an almost
unlimited extent. The repulsion of their particles causes them to
expand, while within certain limits they are easily compressed. This
materially affects the state of density and rarety under which they are
at times exhibited.
It may here be stated that all the laws and properties of liquids
(described under the heads of Hydrostatics and Hydraulics) belong also
to aëriform fluids.
The chemical properties of both liquids and fluids belong peculiarly
to the science of Chemistry, and are, therefore, not to any extent,
considered in this volume.
The air which we breathe is an elastic fluid, surrounding the earth,
and extending to an indefinite distance above its surface, and
constantly decreasing upwards in density. It has already been stated
that the air near the surface of the earth bears the weight of that
which is above it.
Being compressed, therefore, by the weight of that above it, it must
exist in a condensed form near the surface of the earth, while in the
upper regions of the atmosphere, where there is no pressure, it is
highly rarefied. This condensation, or pressure, is very similar to
that of water at great depths in the sea.
Besides the two principal properties, gravity and elasticity, the
operations of which produce most of the phenomena of Pneumatics,
it will be recollected that as air, although an invisible is yet a
material substance, possessing all the common properties of matter, it
possesses also the common property of _impenetrability_.
_The Thermometer is an instrument to indicate the temperature of the
atmosphere._ It is constructed on the principle that heat expands and
cold contracts most substances. The thermometer consists of a capillary
tube, closed at the top and terminating downwards in a bulb. It is
filled with mercury which expands and fills the whole length of the
tube or contracts altogether into the bulb, according to the degree of
heat or cold to which it is exposed. Any other fluid which is expanded
by heat and contracted by cold, may be used instead of mercury.
NOTE.—The terms “rarefaction” and “condensation,” and “rarefied” and
“condensed,” must be clearly understood in this connection. They are
applied respectively to the expansion and compression of a body.
As it has been proved by experiment that 100 cubic inches of air weighs
30-1/2 grains, it will readily be conceived that _the whole atmosphere
exercises a considerable pressure on the surface of the earth_. The
existence of this pressure is shown by the following experiments. On
one end of a stout glass cylinder, about 10 inches high, and open at
both ends, a piece of bladder is tied quite air-tight. The other end,
the edge of which is ground and well-greased, is pressed on the plate
of the air-pump, Fig. 331. As soon as the air in the vessel is rarefied
by working the air-pump, the bladder is depressed by the weight of the
atmosphere above it, and finally bursts with a loud report caused by
the sudden entrance of air.
[Illustration: FIG. 331.]
[Illustration: FIG. 332.]
[Illustration: FIG. 333.]
The preceding experiment only serves to illustrate the downward
pressure of the atmosphere. By means of the _Magdeburg hemispheres_,
Figs. 332 and 333, the invention of which is due to Otto von Guericke,
burgomaster of Magdeburg, it can be shown that the pressure acts in all
directions. This apparatus consists of two hollow brass hemispheres
of 4 to 4-1/2 inches diameter, the edges of which are made to fit
tightly, and are well greased. One of the hemispheres is provided with
a stop-cock, by which it can be screwed on to the air-pump, and on the
other there is a handle. As long as the hemispheres contain air they
can be separated without any difficulty, for the external pressure of
the atmosphere is counterbalanced by the elastic force of the air in
the interior. But when the air in the interior is pumped out by means
of an air-pump, the hemispheres cannot be separated without a powerful
effort.
_The Barometer is an instrument to measure the weight of the
atmosphere_, and thereby to indicate the variations of the weather,
etc. It consists of a long glass tube, about thirty-three inches in
length, closed at the upper end, and filled with mercury. The tube is
then inverted in a cup or leather bag of mercury, on which the pressure
of the atmosphere is exerted. The following experiment, which was first
made in 1643, by Torricelli, a pupil of Galileo, gives an exact measure
of the weight of the atmosphere.
[Illustration: FIG. 334.]
A glass tube is taken, about a yard long and a quarter of an inch
internal diameter, Fig. 334. It is sealed at one end, and is quite
filled with mercury. The aperture, C, being closed by the thumb, the
tube is inverted, the open end placed in a small mercury trough, and
the thumb removed. The tube being in a vertical position, the column of
mercury sinks, and, after oscillating some time, it finally comes to
rest at a height, A, which at the level of the sea is about 30 inches
above the mercury in the trough.
The mercury is raised in the tube by pressure of the atmosphere on the
mercury in the trough. There is no contrary pressure on the mercury in
the tube, because it is closed; but, if the end of the tube be opened,
the atmosphere will press equally inside and outside the tube, and the
mercury will sink to the level of that in the trough. It has been shown
that the heights of two columns of liquid in communication with each
other are inversely as their densities; and hence it follows that the
pressure of the atmosphere is equal to that of a column of mercury the
height of which is 30 inches. If, however, the weight of the atmosphere
diminishes, the height of the column which it can sustain must also
diminish.
_Why a vacuum gauge is graduated in inches instead of in pounds is thus
explained._ Take a tube say 35 inches long, closed at one end, filled
with mercury and inverted with its open end in a bowl containing the
same liquid.
The atmosphere will exert on the surface of the mercury in the bowl
a pressure of about 15 pounds per square inch and this pressure will
be transmitted to that in the tube so that the upward pressure inside
the tube at the level of the mercury in the bowl will be 15 pounds per
square inch.
Below the surface the pressure increases, due to the depth of mercury,
but the weight of mercury inside the tube below the level in the bowl
counteracts the weight of that outside so that the upward pressure per
square inch at the surface line is 15 pounds per square inch inside the
tube no matter how much or little it is submerged. In the upper end of
the tube the mercury has dropped away, leaving a complete vacuum.
NOTE.—Moreover it has the advantage over a scientifically graduated
gauge, which would be graded at 0 for a perfect vacuum and 15, or
more nearly 14.7, for atmospheric pressure, that the inch indication
increases as the vacuum is more complete while the absolute pressure
decreases. The inch of mercury has also the advantage over the pound
as a unit for measuring the degree of vacuum or the difference
between the pressure in the condenser and that of the atmosphere that
_there are twice as many inches in a perfect vacuum as there are
pounds_ so that the gauge can be read more closely without fractional
units. It is easier to say 23 inches than eleven and a half pounds.
The 15 pounds will force the mercury up into the tube until the column
is high enough to balance that pressure. One cubic inch of mercury
weighs about half a pound. It would take two cubic inches to weigh a
pound and a column two inches high to exert a pressure of one pound per
square inch of base, or a column 30 inches high to balance the pressure
of 15 pounds.
[Illustration: FIG. 335.]
[Illustration: FIG. 336.]
If instead of a perfect vacuum there was a pressure of two pounds in
the upper end of the tube the column would have to balance a pressure
of 15-2 = 13 pounds and would be 26 inches high. As the absolute
pressure in the top of the tube gets greater, that is to say, as the
difference between that pressure and that of the atmosphere or the
so-called vacuum gets less, the column of mercury gets lower, and its
height is a measure of the completeness of the vacuum.
_Hero’s fountain_, which derives its name from its inventor, Hero, who
lived at Alexandria, 120 B.C., depends on the elasticity of the air.
It consists of a brass dish, D, Fig. 335, and of two glass globes, M
and N. The dish communicates with the lower part of the globe, N, by
a long tube, B; and another tube, A, connects the two globes. A third
tube passes through the dish, D, to the lower part of the globe, M.
This tube having been taken out, the globe, M, is partially filled with
water; the tube is then replaced and water is poured into the dish. The
water flows through the tube, B, into the lower globe, and expels the
air, which is forced into the upper globe; the air, thus compressed,
acts upon water, and makes it jet out as represented in the figure.
If it were not for the resistance of the atmosphere and friction, the
liquid would rise to a height above the water in the dish equal to the
difference of the level in the two globes.
_The fountain in vacuo_, Fig. 336, shows an interesting experiment
made with the air-pump, and shows the elastic force of the air. It
consists of a glass vessel, A, provided at the bottom with a stop-cock,
and a tubulure which projects into the interior. Having screwed this
apparatus on the air-pump, it is exhausted, and the stop-cock being
closed, it is placed in a vessel of water, R. By opening the stop-cock,
the atmospheric pressure upon the water in the vessel makes it jet
through the tubulure into the interior of the vessel, as shown in the
drawing.
NOTE.—_Reference is hereafter very largely made to the mechanical
use of air as a moving power_, or rather as a means for transferring
power, just as it is transferred by a train of wheelwork. Compressed
air can be employed in this way with great advantage in mines,
tunnels, and other confined situations, where the discharge of steam
would be attended with inconvenience. The work is really done in
these cases by a steam-engine or other prime mover in compressing
the air. In the construction of the Mont Cenis tunnel the air was
first compressed by water-power, and then carried through pipes into
the heart of the mountain to work the boring machines. This use of
compressed air in such situations is also of indirect advantage
in serving not only to ventilate the place in which it is worked,
but also to cool it; for it must be remembered that air falls in
temperature during expansion, and therefore, as its temperature in
the machines was only that of the atmosphere, it must, on being
discharged from them, fall far below that temperature. This fall
is so great that one of the most serious practical difficulties in
working machines by compressed air has been found to be the formation
of ice in the pipes by the freezing of the moisture in the air, which
frequently chokes them entirely up.