The metal aluminium has come into general use with surprising rapidity, and during the last twenty-five or thirty years the amount of this metal produced annually has increased from two or three tons to many thousands of tons. Aluminium occurs naturally in large quantities, in the form of alumina, or oxide of aluminium, but for a long time experimenters despaired of ever obtaining the pure metal cheaply on a commercial scale. The oxides of most metals can be reduced, that is deprived of their oxygen, by heating them with carbon; but aluminium oxide holds on to its oxygen with extraordinary tenacity, and absolutely refuses to be parted from it in this way. One process after another was tried, without success, and cheap aluminium seemed to be an impossibility until about 1887, when two chemists, Hall, an American, and Héroult, a Frenchman, discovered a satisfactory solution of the problem. These chemists, who were then scarcely out of their student days, worked quite independently of one another, and it is a remarkable fact that their methods, which are practically alike, were discovered at almost the same time. The process is an interesting mixture of electrolysis and electric heating. An iron crucible containing a mixture of alumina, fluorspar, and cryolite is heated. The two last-named substances are quickly fused, and the alumina dissolves in the resulting fluid. When the mixture has reached the fluid state, electrodes made of carbon are dipped into it, and a current is passed through; with the result that oxygen is given off at the anode, and metallic aluminium is produced at the cathode, in molten drops. This molten metal is heavier than the rest of the fluid, and so it falls to the bottom. From here it is drawn off at intervals, while fresh alumina is added as required, so that the process goes on without interruption. After the first fusing of the mixture no further outside heat is required, for the heat produced by the passage of the current is sufficient to keep the materials in a fluid state. Vast quantities of aluminium are produced in this way at Niagara Falls, and in Scotland and Switzerland. Most of us are familiar with the substance known as caustic soda. The chemical name for this is sodium hydrate, and its preparation by electrolysis is interesting. Common salt is a chemical compound of the metal sodium and the greenish coloured, evil smelling gas chlorine, its proper name being sodium chloride. A solution of this in water is placed in a vat or cell, and a current is sent through it. The solution is then split up into chlorine, at the anode, and sodium at the cathode. Sodium has a remarkably strong liking for water, and as soon as it is set free from the chlorine it combines with the water of the solution, and a new solution of sodium hydrate is formed. The water in this is then got rid of, and solid caustic soda remains.