When a substance changes from the solid to the liquid, or from the liquid to the gaseous state, heat is required to effect the change; and when heat is not supplied from without to produce the change, the body itself, and everything in contact with it, becomes colder. This is the principle of freezing mixtures. When ice and salt are mixed, the salt (from its tendency to absorb and dissolve in water) will cause the ice to melt. But ice in melting uses up a large quantity of heat, and the result is (since the heat has in this case been withdrawn from the sub-stances themselves) that the mixture is rendered very cold, and water poured into a test-tube and placed in it will freeze. In practice, freezing mixtures are used for producing artificial ice in moderate quantities, and in freezing creams, etc.