Gases are bodies which, unlike solids, have no independent shape, and,
unlike liquids, have no independent volume. Their molecules possess
almost perfect mobility; they are conceived as darting about in all
directions, and are continually tending to occupy a greater space. This
property of gases is known by the names _expansibility_, _tension_, or
_elastic force_, from which they are often called _elastic fluids_.
Gases and liquids have several properties in common, and some in which
they seem to differ are in reality only different degrees of the same
property. Thus, in both, the particles are capable of moving; in gases
with almost perfect freedom; in liquids not quite so freely, owing to
a greater degree of viscosity. Both are compressible, though in very
different degrees.
If a liquid and a gas both exist under the pressure of one atmosphere,
and then the pressure be doubled, the water is compressed by about
the 1/20000 part while the gas is compressed by one-half. In density
there is a great difference; water, which is the type of liquids, is
770 times as heavy as air, the type of gaseous bodies, while under
the pressure of one atmosphere. A spiral spring only shows elasticity
when it is compressed; it loses its tension when it has returned to
its primitive condition. A gas has no original volume; it is always
elastic, or in other words, it is always striving to attain a greater
volume; this tendency to indefinite expansion is the chief property by
which gases are distinguished from liquids.
[Illustration: FIG. 337.]
Matter assumes the solid, liquid, or gaseous form according to the
relative strength of the cohesive and repulsive forces exerted between
their molecules. In liquids these forces balance; in gases repulsion
preponderates.
By the aid of pressure and of low temperatures, the force of cohesion
may be so far increased in many gases that they are readily converted
into liquids, and we know now that with sufficient pressure and cold
they may all be liquified. On the other hand, heat, which increases the
_vis viva_ of the molecules, converts liquids, such as water, alcohol
and ether or gas into the aëriform state in which they obey all the
laws of gases. The aëriform state of liquids is known by the name of
_vapor_, while gases are bodies which, under ordinary temperature and
pressure, remain in the aëriform state.
In describing exclusively the properties of gases, we shall, for
obvious reasons, refer to atmospheric air as their type.
_Expansibility of Gases._ This property of gases, their tendency to
assume continually a greater volume, is exhibited by means of the
following experiment:—A bladder, closed by a stop-cock and about half
full of air, is placed under the receiver of the air pump, Fig. 337,
and a vacuum is produced, on which the bladder immediately distends.
[Illustration: FIG. 338.]
This arises from the fact that the molecules of air flying about in
all directions press against the sides of the bladder. Under ordinary
conditions, this internal pressure is counterbalanced by the air in the
receiver, which exerts an equal and contrary pressure. But when this
pressure is removed, by exhausting the receiver, the internal pressure
becomes evident. When air is admitted into the receiver, the bladder
resumes its original form.
_The compressibility of gases_ is readily shown by the _pneumatic
syringe_, Fig. 338. This consists of a stout glass tube closed at one
end, and provided with a tight-fitting packed piston. When the rod of
the piston is pressed down in the cube, the air becomes compressed into
a smaller volume; but as soon as the force is removed the air regains
its original volume, and the piston rises to its former position.
_Weight of Gases._ From their extreme fluidity and expansibility, gases
seem to be uninfluenced by the force of gravity: they nevertheless
possess weight like solids and liquids. To show this, a glass globe
of 3 or 4 quarts’ capacity is taken, Fig. 339, the neck of which is
provided with a stop-cock, which hermetically closes it, and by which
it can be screwed on the plate of the air-pump.
The globe is then exhausted, and its weight determined by means of a
delicate balance. Air is now allowed to enter, and the globe again
weighed. The weight in the second case will be found to be greater than
before, and if the capacity of the vessel is known the increase will
obviously be the weight of that volume of air.
When the atoms or particles which constitute a body are so balanced
by a system of attractions and repulsions that they resist any force
which tends to change the figure of the body, they will possess a
property, known by the name of elasticity. _Elasticity, therefore, is
the property which causes a body to resume its shape after it has been
compressed or expanded._
[Illustration: FIG. 339.]
_Pressure exerted by Gases._ Gases exert on their own molecules, and
on the sides of vessels which contain them, pressures which may be
regarded from two points of view. First, we may neglect the weight of
the gas; secondly, we may take account of its weight. If we neglect the
weight of any gaseous mass at rest, and only consider its expansive
force, it will be seen that the pressures due to this force act with
the same strength on all points, both of the mass itself and of the
vessel in which it is contained.
It is a necessary consequence of the elasticity and fluidity of gases
that _the repulsive force between the molecules is the same at all
points, and acts equally in all directions_.
If we consider the weight of any gas, we shall see that it gives rise
to pressures which obey the same laws as those produced by the weight
of liquids. Let us imagine a cylinder, with its axis vertical, several
miles high, closed at both ends and full of air. Let us consider any
small portion of the air enclosed between two horizontal planes. This
portion must sustain the weight of all the air above it, and transmit
that weight to the air beneath it, and likewise to the curved surface
of the cylinder which contains it, and at each point in a direction at
right angles to the surface. Thus the pressure increases from the top
of the column to the base; at any given layer it acts equally on equal
surfaces, and at right angles to them, whether they are horizontal,
vertical, or inclined.
The pressure acts on the sides of the vessel, and it is equal to the
weight of a column of gas whose base is this surface, and whose height
its distance from the summit of the column. _The pressure is also
independent of the shape and dimensions of the supposed cylinder_,
provided the height remain the same.
For a small quantity of gas the pressures due to its weight are quite
insignificant, and may be neglected; but for large quantities, like the
atmosphere, the pressures are considerable, and must be allowed for.
_Diffusion of gases._—Liquids mixed together, gradually separate, and
lie superimposed in the order of their densities, and the surfaces of
the separation of the liquids are horizontal. But when gases are mixed,
they present other conditions of equilibrium, as follows.
1.—A homogeneous and persistent mixture is formed rapidly, so that all
parts of the same volume are composed of the same proportions of the
mixed gases.
2.—In a mixture of gases, the pressure (or elastic force), exercised by
each of the gases, is the same as it was when alone.
3.—The rapidity with which the diffusion takes place, varies with the
specific gravity of the gases. The more widely two gases differ in
density, the quicker the process of intermixture.
_Evaporation._—This is the slow formation of vapor from the surface
of a liquid. The elastic force of a vapor which saturates a space
containing a gas (like air), is the same as in a vacuum. The principal
causes which influence the amount and rapidity of evaporation are as
follows.
1st.—_Extent of a surface._ As the evaporation takes place from the
surface, an increase of surface evidently facilitates evaporation.
2d.—_Temperature._ Increasing the elastic force of vapor, has a most
important influence on the rapidity of evaporation; therefore the
temperature of ebullition marks the maximum point of evaporation.
3d.—_The quantity of the same liquid already in the atmosphere_
exercises an important influence on evaporation. The atmosphere can
absorb only a certain amount of vapor, and evaporation ceases entirely
when the air is saturated, but it is greatest when free from vapor,
that is perfectly dry.
4th.—_Renewal of the air._ If currents of air are continually
removing the saturated atmosphere from above the surface of a liquid,
evaporation takes place most rapidly, since new portions of air,
capable of absorbing moisture, are presented to it. Evaporation is
therefore more rapid in a breeze than in still air.
5th.—_Pressure on the surface of the liquid influences evaporation_,
because of the resistance thus offered to the escape of the vapor. That
is to say—water boils more freely in an open vessel than within a steam
boiler under pressure. Hence, the necessity for having large steam
disengaging surfaces to prevent priming or lifting of the water when
the boiler is forced beyond its rated capacity.