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. CHAPTER XII MEASURING THE VELOCITY OF A SHELL In at least two of the preceding chapters of this book reference has been made to the speed at which a shell fired from a gun travels through the air. Such velocities as 3,000 feet per second have been mentioned in this connection, and some readers are sure to have wondered how such measurements could possibly be made. Possibly some sceptics have even supposed that they were not measured at all but simply estimated in some way or other. They are actually measured, however, and by very simple and ingenious means. Needless to say, electricity plays a very important part in this wonderful achievement. In fact, without the aid of electricity it is difficult to see how it could be done at all.