The Romance of Modern Chemistry

METALS, COMMON AND UNCOMMON similar way thermit may be employed for repairing iron shafts and pipes. In aluminium we have an example of a metal the price of which has fallen in a remarkable manner, not because fresh sources of the metal have been discovered, but because the increased demand has led to cheaper and more efficient methods of production. There are other metals, however, which are costly, not because there is any difficulty about their extraction, but because the natural supply is limited. Platinum is a case in point. Endowed with unique and valuable properties, it is comparatively rare, and possibly the reader has never seen a specimen; it costs nearly twice as much as gold. Platinum is a silvery metal, twenty-one times as heavy as water, bulk for bulk, and does not rust or tarnish. Like gold, it is a “ noble ” metal, and is not dissolved by any single acid which we know, not even by aqua fortis (nitric acid). A mixture of nitric and hydrochloric acids, however, will dissolve both platinum and gold, and it was the power of this mixture to attack the latter metal which led the alchemists to speak of it as “ aqua regia.'" Platinum is in a special sense the chemist’s metal. Its very high melting-point — 3200° Fahrenheit — and its chemical inertness make it valuable to him, and platinum crucibles are to be reckoned among the indispensables of a properly-equipped chemical laboratory. There is another characteristic of platinum which to the casual reader may seem most insignificant, but which, as it turns out, is of the greatest importance in a certain manufacture. This is the fact that with rising tem- perature platinum expands at nearly the same rate as glass. Why should that be of any consequence ? the 68