The Romance of Modern Chemistry

METALS, COMMON AND UNCOMMON oxide which acts as a protective layer, the powdered metal burns vigorously, like magnesium, when it is heated, and in this way a very high temperature is produced. If the oxygen which is necessary for the combustion of the aluminium is mixed with the metal at the start, instead of coming from the air as the burning proceeds, an even higher temperature can be reached. But how, the reader may ask, can we mix a gas with a solid ? In the literal sense, certainly, this cannot be done, for to burn half an ounce of aluminium powder as much as fifteen pints of oxygen gas would be required. But the oxygen may be mixed with the aluminium in a compact or condensed condition in the form of some compound, out of which the aluminium has no difficulty in extracting it. Iron oxide is such a compound, so if a mixture of powdered aluminium and iron oxide, known as “ thermit,” is ignited at one point an action sets in which spreads through the whole mass, giving out intense heat, and resulting in the formation of aluminium oxide and molten metallic iron. The aluminium, in fact, feeds on the oxygen of which it has deprived the iron. The heat produced in this competition for the oxygen is so intense that if some thermit mixture is placed on an iron plate half an inch thick, and ignited, a hole is melted in the plate. The heat stored up in thermit may, however, be turned to more practical account in the following interesting manner:—If the ends of two steel rails are pressed to- gether and some of the intensely hot fluid iron produced in the thermit reaction is run out of a crucible on to the junction, the crevices are filled, the heat is such that the ends of the rails are softened, and may be welded by the applied pressure so that a sound joint is made. In a 67