Except in the case of alluvial ores which have been broken already by the action of ancient streams of water, nearly all ores (except iron) have to be crushed before the ores can be separated out. Some of this work is done by the very simplest contrivances, showing how in some cases invention has almost come to a stop through the machines having been reduced to their simplest form. A notable instance of this is the stamp mill, in which heavy timbers are lifted up by machinery and then allowed to slide down upon the ore, just like gigantic pestles. More elaborate grinding machines are sometimes used, however, but it is impossible to mention them all here. The action of sorting out the fragments of ore from the miscellaneous assortment of crushed rocks is termed "concentrating," and the sorted ores are called "concentrates." Another metal which has proved itself of immense importance in war is aluminium, and it fittingly comes at the close of the list since it is dealt with in a manner peculiar to itself. Practically all the others are obtained from their ores by means of heat and heat alone. Aluminium is obtained by electricity acting in the process called electrolysis. 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.