r 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 deg. C. is suddenly compressed to one
half its original volume, its temperature rises 87 deg. C.

91. Cooling by Expansion. If a gas expands suddenly, its temperature
falls; for example, if a mass of gas at 87 deg. C. is allowed to expand
rapidly to twice its original volume, its temperature falls to 0 deg. 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 deg. C. below
zero would have to be obtained in