The Romance of Modern Chemistry

BELOW ZERO globe as being normally about 250° Fahrenheit, then water would exist only in the form of vapour or steam, and in order to liquefy it we should have to bring the temperature below 212°, the boiling-point of water. At any temperature lower than 212° steam will condense under the ordinary pressure of the atmosphere. Now we must remember that every other liquid, has its own boiling-point, and substances which we know as gases are simply liquids whose boiling-points are at a tempera- ture lower than that prevailing on the surface of the globe. Sulphur dioxide, for example, the colourless, choking gas which is produced when sulphur is burned, is very easily obtained as a liquid at temperatures not much below the freezing-point of water. The boiling-point of this liquid sulphur dioxide is 18° Fahrenheit under the ordinary pressure, so that when the gas is passed through a tube surrounded by a freezing mixture of ice and salt, it condenses to the liquid form just as steam would do if it were passed through a tube surrounded by cold water. It is, in fact, quite easy to obtain liquid sulphur dioxide, and it is now sold in syphons, just as if it were so much soda water. When we come to gases like ammonia and carbon dioxide, which are less easily condensed, it is found advis- able to use high pressure as an aid to liquefaction. The reader will understand the object of this if he remembers that the boiling-point of a liquid gradually rises with the pressure to which it is exposed. For example, water boils at 212° Fahrenheit under a pressure of one atmos- phere, but at 250° when the pressure is two atmospheres. Conversely, then, when a gas is kept under high pressure, less cooling is necessary to bring it below its boiling-point. 184