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