If the oft-quoted "man in the street" were asked the two commonest things on earth he might possibly name oxygen as one, and so far he would be right, but the chances are much against his naming aluminium as the second. If he did not, however, he would be wrong. Aluminium and oxygen form alumina, of which are constituted the sapphire, the ruby and other precious stones, but alumina is most commonly found in combination with silica, or silicon and oxygen. This compound is called silicate of aluminium, and of it are formed clay and many rocks. The reason why the metal aluminium was until recently rare and expensive was because of the great difficulty of disentangling the metal from this rather complex combination. And these two commonest elements have, under certain conditions, a rare affinity for each other. They join forces with such energy that great heat is given out in the process. This, again, we may regard as an example of the conservation of energy. Heat had to be used up, apparently, in separating the aluminium and oxygen as they were found together in the natural state. And that heat reappears when they combine together again. This is a most useful principle, for if heat has disappeared anywhere in the course of some operation, we know that in all probability, if we go about it the right way, we can get that heat back again, perhaps in a more convenient form. That is so in this case at all events. Now aluminium will not readily combine with atmospheric oxygen, but it will readily do so with oxygen from the oxide of a metal. So if we put into a vessel some oxide of iron and some finely powdered aluminium, and give it some heat at one point, just to set the process going, the whole mass will burn with intense heat. And when the burning is finished the crucible will be found to contain (1) some molten iron, the oxide of iron with the oxygen gone, and (2) some oxide of aluminium or alumina, in the form which we call corundum, a very hard substance which in a powdered form is used for grinding hard metals. We start, you will notice, with a pure metal and an oxide. We finish with a pure metal and an oxide, only the oxygen has changed its quarters, having passed from the iron to the aluminium. And in the course of the change a vast amount of pent-up heat has been liberated. Aluminium is thus a fuel, strange though it may seem to say so, just as coal is. Coal, however, is willing to pair off with oxygen from the air, while aluminium, more fastidious, will only accept it as partner when it can steal it from another combination. But the practical result is eminently satisfactory, for the action of the aluminium and iron oxide is to leave us with a crucible full of molten iron at a very high temperature. And this can be used in various ways.