GENERAL PROPERTIES OF GASES
85. Bicycle Tires. We know very well that we cannot put more than a
certain amount of water in a tube, but we know equally well that the
amount of air which can be pumped into a bicycle or automobile tire
depends largely upon our muscular energy. A gallon of water remains a
gallon of water and requires a perfectly definite amount of space, but
air can be compressed and compressed, and made to occupy less and less
space. While it is true that air is easily compressed, it is also true
that air is elastic and capable of very rapid and easy expansion. If a
puncture occurs in a tire, the compressed air escapes very quickly;
that is, the compressed air within the tube has taken the first
opportunity offered for expansion.
[Illustration: FIG. 51.--By squeezing the bulb, air is forced out of
the nozzle.]
The fact that air is elastic has added materially to the comfort of
the world. Transportation by bicycles and automobiles has been greatly
facilitated by the use of air tires. In many hospitals, air mattresses
are used in place of hair, feather, or cotton mattresses, and in this
way the bed is kept fresher and cleaner, and can be moved with less
danger of discomfort to the patient. Every time we squeeze the bulb of
an atomizer, we force compressed or condensed air through the
atomizer, and the condensed air pushes the liquid out of the nozzle
(Fig. 51). Thus we see that in the necessities and conveniences of
life compressed air plays an important part.
86. The Danger of Compression. Air under ordinary atmospheric
conditions exerts a pressure of 15 pounds to the square inch. If, now,
large quantities of air are compressed into a small space, the
pressure exerted becomes correspondingly greater. If too much air is
blown into a toy balloon, the balloon bursts because it cannot support
the great pressure exerted by the compressed air within. What is true
of air is true of all gases. Dangerous boiler explosions have occurred
because the boiler walls were not strong enough to withstand the
pressure of the steam (which is water in the form of gas). The
pressure within the boilers of engines is frequently several hundred
pounds to the square inch, and such a pressure needs a strong boiler.
87. How Pressure is Measured in Buildings. In the preceding Section
we saw that undue pressure of a gas may cause explosion. It is
important, therefore, that authorities keep strict watch on gases
confined within pipes and reservoirs, never allowing the pressure to
exceed that which the walls of the reservoir will safely bear.
[Illustration: FIG. 52.--A pressure gauge.]
Pressure in a gas pipe may be measured by a simple instrument called
the pressure gauge: The gauge consists of a bent glass tube containing
mercury, and so made that one end can be fitted to a gas jet (Fig.
52). When the gas cock is closed, the mercury stands at the same level
in both arms, but when the cock is opened, the gas whose pressure is
being measured forces the mercury up the opposite arm. If the pressure
of the gas is small, the mercury changes its level but very little. It
is clear that the height of a column of mercury is a measure of the
gas pressure. Now it is known that one cubic inch of mercury weighs
about half a pound. Hence a column of mercury one inch high indicates
a pressure of about one half pound to the square inch; a column two
inches high indicates a pressure of about one pound to the square
inch, and so on.
This is a very convenient way to measure the pressure of the
illuminating gas in our homes and offices. The gauge is attached to
the gas burner and the pressure is read by means of a scale attached
to the gauge. (See Laboratory Manual.)
In order to have satisfactory illumination, the pressure must be
strong enough to give a steady, broad flame. If the flame from any gas
jet is flickering and weak, it is usually an indication of
insufficient pressure and the gas company should investigate
conditions and see to it that the consumer receives his proper value.
87. The Gas Meter. Most householders are deeply interested in the
actual amount of gas which they consume (gas is charged for according
to the number of cubic feet used), and therefore they should be able
to read the gas meter which indicates their consumption of gas. Such
gas meters are furnished by the companies, and can be read easily.
[Illustration: FIG. 53.--The gas meter indicates the number of cubic
feet of gas consumed.]
The instrument itself is somewhat complex. It will suffice to say that
within the meter box are thin disks which are moved by the stream of
gas that passes them. This movement of the disks is recorded by
clockwork devices on a dial face. In this way, the number of cubic
feet of gas which pass through the meter is automatically registered.
89. The Relation between Pressure and Volume. It was long known that
as the pressure of a gas increases, that is, as it becomes compressed,
its volume decreases, but Robert Boyle was the first to determine the
exact relation between the volume and the pressure of a gas. He did
this in a very simple manner.
Pour mercury into a U-shaped tube until the level of the mercury in
the closed end of the tube is the same as the level in the open end.
The air in the long arm is pressing upon the mercury in that arm, and
is tending to force it up the short arm. The air in the short closed
arm is pressing down upon the mercury in that arm and tending to send
it up the long arm. Since the mercury is at the same level in the two
arms, the pressure in the long arm must be equal to the pressure in
the short arm. But the long arm is open, and the pressure in that arm
is the pressure of the atmosphere. Therefore the pressure in the short
arm must be one atmosphere. Measure the distance _bc_ between the top
of the mercury and the closed end of the tube.
[Illustration: FIGS. 54, 55.--As the pressure on the gas increases,
its volume decreases.]
Pour more mercury into the open end of the tube, and as the mercury
rises higher and higher in the long arm, note carefully the decrease
in the volume of the air in the short arm. Pour mercury into the tube
until the difference in level _bd_ is just equal to the barometric
height, approximately 32 inches. The pressure of the air in the closed
end now supports the pressure of one atmosphere, and in addition, a
column of mercury equal to another atmosphere. If now the air column
in the closed end is measured, its volume will be only one half of its
former volume. By doubling the pressure we have reduced the volume one
half. Similarly, if the pressure is increased threefold, the volume
will be reduced to one third of the original volume.
90. Heat due to Compression. We saw in Section 89 that whenever the
pressure exerted upon a gas is increased, the volume of the gas is
decreased; and that whenever the pressure upon a gas is decreased, the
volume of the gas is increased. If the pressure is changed very
slowly, the change in the temperature of the gas is imperceptible; if,
however, the pressure is removed suddenly, the temperature falls
rapidly, or if the pressure is applied suddenly, the temperature rises
rapidly. When bicycle tires are being inflated, the pump becomes hot
because of the compression of the air.
The amount of heat resulting from compression is surprisingly large;
for example, if a mass of gas at 0° C. is suddenly compressed to one
half its original volume, its temperature rises 87° C.
91. Cooling by Expansion. If a gas expands suddenly, its temperature
falls; for example, if a mass of gas at 87° C. is allowed to expand
rapidly to twice its original volume, its temperature falls to 0° C.
If the compressed air of a bicycle tire is allowed to expand and a
sensitive thermometer is held in the path of the escaping air, the
thermometer will show a decided drop in temperature.
The low temperature obtained by the expansion of air or other gases is
utilized commercially on a large scale. By means of powerful pistons
air is compressed to one third or one fourth its original volume, is
passed through a coil of pipe surrounded with cold water, and is then
allowed to escape into large refrigerating vaults, which thereby have
their temperatures noticeably lowered, and can be used for the
permanent storage of meats, fruits, and other perishable material. In
summer, when the atmospheric temperature is high, the storage and
preservation of foods is of vital importance to factories and cold
storage houses, and but for the low temperature obtainable by the
expansion of compressed gases, much of our food supply would be lost
to use.
92. Unexpected Transformations. If the pressure on a gas is greatly
increased, a sudden transformation sometimes occurs and the gas
becomes a liquid. Then, if the pressure is reduced, a second
transformation occurs, and the liquid evaporates or returns to its
original form as a gas.
In Section 23 we saw that a fall of temperature caused water vapor to
condense or liquefy. If temperature alone were considered, most gases
could not be liquefied, because the temperature at which the average
gas liquefies is so low as to be out of the range of possibility; it
has been calculated, for example, that a temperature of 252° C. below
zero would have to be obtained in order to liquefy hydrogen.
Some gases can be easily transformed into liquids by pressure alone,
some gases can be easily transformed into liquids by cooling alone; on
the other hand, many gases are so difficult to liquefy that both
pressure and low temperature are needed to produce the desired result.
If a gas is cooled and compressed at the same time, liquefaction
occurs much more surely and easily than though either factor alone
were depended upon. The air which surrounds us, and of whose existence
we are scarcely aware, can be reduced to the form of a liquid, but the
pressure exerted upon the portion to be liquefied must be thirty-nine
times as great as the atmospheric pressure, and the temperature must
have been reduced to a very low point.
93. Artificial Ice. Ammonia gas is liquefied by strong pressure and
low temperature and is then allowed to flow into pipes which run
through tanks containing salt water. The reduction of pressure causes
the liquid to evaporate or turn to a gas, and the fall of temperature
which always accompanies evaporation means a lowering of the
temperature of the salt water to 16° or 18° below zero. But immersed
in the salt water are molds containing pure water, and since the
freezing point of water is 0° C, the water in the molds freezes and
can be drawn from the mold as solid cakes of ice.
[Illustration: FIG. 56.--Apparatus for making artificial ice.]
Ammonia gas is driven by the pump _C_ into the coil _D_ (Fig. 56)
under a pressure strong enough to liquefy it, the heat generated by
this compression being carried off by cold water which constantly
circulates through _B_. The liquid ammonia flows through the
regulating valve _V_ into the coil _E_, in which the pressure is kept
low by the pump _C_. The accompanying expansion reduces the
temperature to a very low degree, and the brine which circulates
around the coil _E_ acquires a temperature below the freezing point of
pure water. The cold brine passes from _A_ to a tank in which are
immersed cans filled with water, and within a short time the water in
the cans is frozen into solid cakes of ice.