The Romance of Modern Chemistry

BELOW ZERO metal. To the case of ice and salt a similar rule applies. Every schoolboy knows that pure water freezes at 32° Fahrenheit (0° Centigrade), but it is a curious fact that water containing salt does not freeze until a lower tem- perature has been reached. That means that a mixture of snow and a little solid salt should, strictly speaking, be in the liquid condition at 32° Fahrenheit; there cannot therefore be true equilibrium between snow and salt at this temperature. Now, in Nature, things are always trying to get into the most stable condition possible, in other words, to reach their true equilibrium. Water finds its own level, a hot and a cold object put side by side gradually and of their own accord assume the same temperature, while positive and negative electricity unite whenever they get the opportunity. Similarly snow and salt, when mixed together at 32° Fahrenheit, do their best to get into that condition which Nature has prescribed as the most stable one for them at that temperature; the result is that the snow melts and the salt dissolves in the melted ice. Now both these processes use up heat; as they take place spontaneously, this heat is taken from the surround- ings, and the temperature of the mixture and of the con- taining vessel falls. The reader will at once admit that heat is required to melt snow, and he will see that the addition of salt is an ingenious way of persuading the snow to melt, and so to abstract a definite amount of heat from its surroundings. For the same quantity of heat is always required to melt a pound of snow, whatever be the way in which we cause the melting to take place. So far as we have gone, then, methods of producing cold depend either on dissolving a solid in a liquid, or on making a solid melt by a little scientific stratagem. But 182