Although this trick is performed by means of chemicals, yet its product is obtained really by the use of mechanical laws. We must remember that ice is exactly the same thing as water so far as its composition is concerned, differing only in its state of density.

Ice, water, and steam differ in density through the possession of a greater or less quantity of heat. Hence, the turning of water into ice really is a case of the operation of mechanical laws.

Now for the experiment. Put into a wide-mouthed jam-jar a smaller glass vessel containing the water to be frozen. Around the latter put a mixture of sulphate of soda (Glauber’s salt) and hydrochloric acid (spirits of salts). The proportions must be eight parts of the former to five of the latter.

The action of these two chemicals on one another is to cause a cold of fifteen to seventeen degrees below zero, or forty-seven degrees below freezing point.

The same result may be obtained by mixing equal parts of nitrate of ammonia and water. In winter-time when the snow is on the ground, with a mixture of one part snow and one part common table salt an intense cold of twenty degrees below zero is obtained.

From this last fact we see how stupid are those people who sprinkle the salt on the pavements to get rid of the snow. True, the latter melts, but only after the production of intense cold, which is the cause of many diseases, not only slight ones like colds and chilblains, but too often the forerunners of consumption and other lung troubles.