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 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 spaces called pores; and that when a gas is condensed, the pores become smaller, and that when a gas expands, the pores become larger.