It is surprising to learn that aluminium is one of the very commonest things on the face of the earth. Clay and many common rocks are very largely made of it. Clay, to be precise, is a silicate of alumina, a term which is interesting when it is explained. Silica is the name given to oxide of silicon. Sand is mostly silica. Alumina, too, is oxide of aluminium. Silicate of alumina is a combination of the two.
Any clay, therefore, could be used as an ore from which to obtain aluminium, but of course there are certain minerals specially suitable for the purpose, since in them the metal is plentiful and easily extracted.
In another chapter reference is made to the production of caustic soda from a solution of common salt by electrolysis. The same principle, precisely, is used to obtain the metal aluminium from its ore, which is generally an oxide.
Common salt, let me remind you, is sodium and chlorine combined. When you dissolve it in water it becomes ionized, which means that each molecule of salt splits up into two ions one of which is electrically positive and the other electrically negative. Then, when we introduce two electrodes into the solution and connect them to a battery or dynamo, all the positive ions go to one electrode and all the negative ions to the other.
We cannot dissolve aluminium ore in water, but we can in a bath of molten cryolite, and for some reason or other, whether because of the heat or not we cannot say, the ore becomes ionized, the aluminium atoms being one sort and the oxygen atoms the other sort. These ions then sort themselves out, the oxygen ions being taken into combination with the carbon rod which forms the positive electrode, while the metal ions collect upon the negative electrode. Since this latter is a slab of carbon which forms the bottom of the vessel in which the process is carried on, the result is that pure aluminium gradually accumulates in the bottom of
the vessel and can be drawn off from time to time.
Aluminium is always produced in places where electric power can be obtained cheaply, such as near waterfalls.