Now take ammonia gas. The "spirits of hartshorn" we buy at the chemist's is water impregnated with this gas. At ordinary living temperatures the water gives out the gas, as a sniff at the bottle proves in a most effective manner.
If this gas were cooled to 37·3° below zero it would assume a liquid state, i.e. that temperature marks its boiling point. Similarly steam, cooled to 212° Fahr., becomes water. Boiling point, therefore, merely means the temperature at which the change occurs.
Ammonia liquid, when gasifying, absorbs a great amount of heat from its surroundings—air, water, or whatever they may be. So that if we put a tumbler full of the liquid into a basin of water it would rob the water of enough heat to cause the formation of ice.
The refrigerating machine, generally employed on ships, is one which constantly turns the ammonia liquid into gas, and the gas back into liquid. The first process produces the cold used in the freezing-rooms. The apparatus consists of three main parts:—
(1) The compressor, for squeezing ammonia gas.
(2) The condenser, for liquefying the gas.
(3) The evaporator, for gasifying the liquid.
The compressor is a pump. The condenser, a tube or series of tubes outside which cold water is circulated. The evaporator, a spiral tube or tubes passing through a vessel full of brine. Between the condenser and evaporator is a valve, which allows the liquid to pass from the one to the other in proper quantities.
We can now watch the cycle of operations. The compressor sucks in a charge of very cold gas from the evaporator, and squeezes it into a fraction of its original volume, thereby heating it. The heated gas now passes into the condenser coils and, as it expands, encounters the chilling effects of the water circulating outside, which robs it of heat and causes it to liquefy.