The simplest ice-machine is a hermetically-sealed bottle connected with an air-pump. Exhaust the air, and the water begins to boil and to grow cold. As the air is drawn off, the water begins to freeze; and if—by an ingenious device—the steam that it generates is absorbed into a reservoir of sulphuric acid, or any other substance which has a great affinity for watery vapor, a good quantity of ice is obtained. This is the practical use of liquefying gases; naturally, they all boil at temperatures much below that of the air, in which they exist in the vaporised state that follows after boiling. Take, therefore, your liquefied gas; let it boil and give off its steam. This steam, absorbing by its expansion all the surrounding heat, may be used to make ice, to cool beer-cellars, to keep meat fresh all the way from New Zealand, or—as has been largely done at Suez—to cool the air in tropical countries. Put pressure enough on your gas to turn it into a liquid state, at the same time carrying away by a stream of water the heat that it gives off in liquefying. Let this liquid gas into a “refrigerator,” where it boils and steams, and draws out the heat; and then by a sucking-pump drive it again into the compressor, and let the same process go on ad infinitum, no fresh material being needed, nothing, in fact, but the working of the pump. Sulphurous acid is a favorite gas, ammonia is another; and—besides the above practical uses—they have been employed in a number of startling experiments.