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. The fact that certain substances are soluble, like sugar in water, shows that the molecules of sugar find a lodging place in the spaces or pores between the molecules of water, in much the same way that pebbles find lodgment in the chinks of the coal in a coal scuttle. An indefinite quantity of sugar cannot be dissolved in a given quantity of liquid, because after a certain amount of sugar has been dissolved all the pores become filled, and there is no available molecular space. The remainder of the sugar settles at the bottom of the vessel, and cannot be dissolved by any amount of stirring. If a piece of potassium permanganate about the size of a grain of sand is put into a quart of water, the solid disappears and the water becomes a deep rich red. The solid evidently has dissolved and has broken up into minute particles which are too small to be seen, but which have scattered themselves and lodged in the pores of the water, thus giving the water its rich color.