The Romance of Modern Chemistry

BELOW ZERO by them, so that when a highly compressed gas is allowed to expand, there is a social force resisting the separation of the molecules which is involved in the expansion. In overcoming this social force, work must be done, and for the performance of this work heat is required. This heat is taken mostly from the gas itself, which there- fore exhibits the phenomenon of “ self-cooling. All this may be put more definitely and practically by saying that when highly compressed air is allowed to expand through a small nozzle or a porous plug, it becomes slightly colder. In the actual machines for making liquid air the device is further adopted of allow- ing the expanded and slightly cooled air to circulate round the coil of tubing through which the next lot of compressed air is approaching the nozzle. In such a regenerative process the cooling effects arc accumulated, and the air which circulates through the machine, alter- nately compressed and expanded, becomes gradually cooler until at length it condenses and drops into a vessel placed to receive it. A vessel which is to contain liquid air or liquid hydrogen must be specially constructed if it is to be of any use at all. If we were to put liquid air, which boils at —347° Fahrenheit, in an ordinary glass vessel we should very shortly see the last of it, owing to the heat communicated through the walls of the vessel. That is, in fact, exactly what would happen if we put a glass of water in a hot-air bath kept at 400° or 500°. Such a communication of heat, however, may be very much diminished, as Professor Dewar has shown, by using double-walled vessels and removing the air from the space between the walls. Sections of two such vessels, a tube and a flask, are shown in Fig. 9. 187