The Romance of Modern Chemistry

METALS, COMMON AND UNCOMMON allowed to cool slowly, is said to be “tempered to the yellow,1’ and is hard enough to take a fine cutting edge. It must be remembered that steel which has been hardened without being tempered is of no use for ordinary purposes ; it is too brittle. It is many centuries now since man first began to discover the valuable properties of iron, and the passage of time has only led to a gradually widened range of application, and to improved methods of production. One can name metals, however, which for a long time after their discovery were regarded as curiosities, and have only recently and more or less suddenly been in large demand as their useful properties have been realised. Aluminium is a notable example of this. Fifty or sixty years ago it cost twenty shillings an ounce; now it can be purchased for less than a shilling a pound. The very high price of the metal was due, not to scarcity of material from which aluminium could be produced, but to the fact that there was little or no demand for it, and no satisfactory method of extract- ing it from its ores. As a matter of fact, aluminium is one of the most common constituents of the earth’s crust, occurring in the combined state as mica, felspar, clay, and slate. It is in many respects a remarkable metal. It is exceedingly light, and yet, unlike most other metals of this class, is not easily tarnished even in the presence of moisture. On account of its lightness it is extensively used in military fittings, while its resistance to the action of animal and vegetable juices renders it serviceable in the manufacture of cooking utensils. Although aluminium in the mass is not easily oxidised in air, probably because it gets coated with a thin film of 66