Tramway rails, for example, can be joined together by it. A mould is formed around the ends of two rails, where they "butt" together, and into this mould a quantity of the melted iron can be poured. So hot is it that it partially melts the ends of the rails, and then, amalgamating with them, it forms a perfectly homogeneous connection between them. The same method can be applied to the repair of iron structures of all kinds. The propeller shaft of a ship, for example, sometimes breaks on a voyage. Such a catastrophe is fraught with the most serious consequences, unless it can be quickly repaired. Thermit, as this process is called, is perhaps the only means whereby, under certain conditions, this can be accomplished. The extraordinary heat of the metal produced in this way is demonstrated by the fact that if it be poured on to an iron plate an inch thick it goes clean through it. It melts its way through instantly. But although such high temperatures are at the command of the modern manufacturer, there are some things--indeed many things--which still baffle him, the diamond, for example. It is true that diamonds of small size have been made, but larger ones have so far defied all efforts. One very interesting fact about this may be mentioned in concluding this chapter. Sir Andrew Noble, a member of the great firm of Armstrong, Whitworth & Co., of Elswick, tried the experiment of exploding some cordite, a high explosive, inside a steel vessel of enormous strength. He thus produced what is believed to be the highest temperature ever produced on earth. It is reckoned to have been 5200° C., and the pressure at the same time was, it is calculated, 50 tons per square inch. His intention was not to make diamonds, but Sir William Crookes predicted that diamonds would be the result. For the cordite consisted mainly of carbon, which, as is well known, is the material of which the diamond is formed, and the combination of high temperature and high pressure is just what is needed, so it is believed, to bring the carbon into this particular form. And true enough, on the iron being examined after the explosion, there were seen tiny diamonds. For larger ones even higher temperatures and greater pressures are, no doubt, necessary, and as the diamond, like gold, has a peculiar fascination for mankind, so the efforts to manufacture it will continue. In years to come the means may be found of creating these extreme conditions of temperature and pressure, and so another of the problems of the ages will be solved. [Illustration: _By permission of the British Aluminium Co_ A STRIKING FEATURE OF MODERN ALUMINIUM WORKS