The Romance of Modern Chemistry

TWO METALS BETTER THAN ONE which separates out when the fused alloy begins to solidify is either pure A or pure B, never a compound of the two metals. In the cases where the two metals do form a com- pound, the curve showing the variation of freezing-point with composition is of a different character; there is then an intermediate branch of the freezing-point curve, shaped more or less like a camel’s hump (see Fig. 1Z>). The temperature corresponding to the summit of the hump is the freezing-point of the compound winch is formed, and the composition of the alloy which has this maximum freezing-point gives the composition of the compound. It is interesting now to find that the existence of compounds of mercury and sodium, and of gold and aluminium, which we suspected from their behaviour on mixing, is confirmed by a study of the freezing-point curves for alloys of these metals. In the case of mercury and sodium the freezing-point curve has a hump the top of which is far above the freezing-point of either constituent, so that the existence of a compound is here proved very definitely. The freezing-point curve for alloys of gold and alu- minium has actually two separate humps, showing that these metals combine to form two compounds with different proportions of the constituents. One of the humps corresponds to the formation of the beautiful purple alloy already referred to, and it is very remark- able that this compound, containing 20 per cent, of aluminium, should melt at the same temperature as pure gold. Another method of investigating the nature of alloys which has recently been employed with success is the 80