The Romance of Modern Chemistry

BELOW ZERO just as we can utilise the change of solid into liquid as a means of reaching lower temperatures, so we can employ another change of state for the same purpose—the change, namely, in which a liquid passes into the condition of a vapour. We usually convert a liquid into a gas or vapour by heating it; for the conversion of water at 212° Fahrenheit into steam at 212°, heat is as necessary as it is for the conversion of ice or snow at 32° Fahren- heit into water at the same temperature. Evaporation, then—that is, the process by which a liquid is changed into a vapour—only takes place when heat is supplied. If by any means we can cause evaporation to take place without the external application of heat, then the neces- sary heat will be taken from the evaporating liquid itself and its surroundings. Under these circumstances evaporation produces cold. A very simple way of causing a volatile liquid to eva- porate rapidly without heating is to blow a strong current of air through it. That by this method a considerable reduction of temperature may take place can be shown by a very simple experiment. A small pool of water is made on the top of a flat wooden block, and in this pool is set a flask containing strong ammonia solution. A strong current of air is then blown through the liquid with the aid of a bellows; the ammonia evaporates rapidly, and before long the flask is frozen hard to the block. With these two general ways of producing cold at their disposal, Faraday and other chemists after him have been able to obtain in the liquid state many substances which exist ordinarily as invisible gases. The point to which the temperature of a gas must be lowered before it begins to liquefy will, of course, vary from one case to another. If we could imagine the temperature of our 183