The Romance of Modern Chemistry

TWO METALS BETTER THAN ONE and the molten mass is raised to incandescence. Such an occurrence may be taken as evidence that chemical com- bination has taken place. Again, the colour of some alloys is markedly different from that of the constituents. Silver and zinc are both white metals, and yet they form a beautiful pink alloy. Gold and aluminium also furnish us with an illustration, for when they are mixed in certain pro- portions a brilliant purple alloy is obtained, quite dis- tinct in colour from both constituents. In spite of these examples, however, it must be said that we do not observe in the formation of alloys generally such a thorough-going change of properties as commonly results from chemical combination. The question, therefore, of mechanical mixture v. chemical compound is not so easily decided. Modern investigators have tackled the problem by studying the freezing-points of alloys in their relation to the freezing-points of the constituent metals, and in the course of these investigations many interesting results nave been brought to light. It is almost the invari- able rule that when a little of a metal B is added to a metal A, the fused alloy begins to solidify at a lower temperature than pure A; it is said to have a lower freezing-point. As we go on adding more and more of the metal B, the alloys produced have lower and lower freezing-points. The same series of phenomena is observed when we add increasing quantities of the metal A to the pure metal B. Suppose we try to picture these results graphically, that is, with the help of a curve. In doing this we represent the composition of the alloy by distances measured along a horizontal line, while the tempera- 78