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.




CHAPTER IX

INVISIBLE OBJECTS


94. Very Small Objects. We saw in Section 84 that gases have a