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 deg. or 18 deg. below zero. But immersed
in the salt water are molds containing pure water, and since the
freezing point of water is 0 deg. 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.

CHAPTER IX

INVISIBLE OBJECTS

94. Very Small Objects. We saw in Section 84 that gases have a
tendency to expand, but that they can be compressed by the application
of force. This observation has led scientists to suppose that
substances are composed of very minute particles called molecules,
separated by small spac