XIII. On certain mechanical properties of metals considered in relation to the periodic law

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Abstract
The influence exerted by a small quantity of metallic or other impurity on a mass of metal is shown by a remarkable series of phenomena the nature of which has hitherto been but little studied, although the effect produced by the presence of such added matter is widely recognised by metallurgists. There are many cases in which a small quantity of impurity has so entirely altered the appearance and the physical properties of a metal as to lead, in the absence of other evidence, to its being mistaken for a distinct elemental substance. The valuable mechanical properties conferred upon metals by associating them with small, but definite, amounts of other metals constitute the main reason why metals devoted to industrial use are seldom employed in a state of purity. A familiar instance of the influence of a small quantity of a metalloid on a mass of metal is presented by the extraordinary change in the properties of pure iron which attends the introduction into the metal of a small quantity of carbon. There is no fact in metallurgy of which the importance is more widely recognised, and when Bergman, in 1781, experimentally demonstrated that the differences between pure iron, steel, and cast-iron depend on the presence or absence of carbon, he expressed his astonishment at the smallness of the amount of carbon capable of producing such effects, and he stated th at the explanation of the phenomenon presented a “difficulty of difficulties”; and the problem has certainly not been solved in the century which has elapsed since Bergman wrote. In other directions the evidence as to the importance of the action of traces of impurity is just as strong. This is indicated by the fact, referred to by Sir Hussey Vivian, that “one thousandth part of antimony converts first-rate ‘best selected’ copper into the worst conceivable,’ and by the observation of Mr. Preece, that “a submarine cable made of the copper of to-day,” the necessity for employing pure metal being recognised, “will carry twice the number of messages that a similar cable of copper would in 1858,” when less importance was attached to the presence of foreign matter in the copper. It may be well to refer to a but little known case in which the change in the structure of a metal produced by the presence of a minute quantity of foreign matter becomes at once evident by comparing the fractured surfaces of the pure and impure masses. Bismuth, when pure, has a fracture which shows large brilliant mirror-like crystalline planes; but, if only the 1/1000 th part of tellurium be present, the fracture is, as a specimen submitted to the Society showed, entirely different, being minutely crystalline and lighter in colour than pure bismuth.

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