Page:Proceedings of the Royal Society of London Vol 60.djvu/166

Rh tions in detail, and they conclude the first part of the paper by stating that they do not contest in any way, as their previous publications abundantly prove, the importance of the part which may be played in the mechanical properties of the alloys by the residues which remain liquid after the main mass of the alloy has solidified, the alloys beingtested either at the ordinary temperature or when heated. But, in order that it may he possible for such cements to intervene and affect the mechanical properties of alloys, the cements must at least have a real existence. Nothing indicates that they do exist in ten out of twelve of the alloys examined. The authors do not express themselves too positively on this point, for some new method of etching may reveal new facts. The impurities which are sought for may happen to concentrate themselves beyond the particular region which has been sectioned. These are, however, gratuitous suppositions. Polishing only indicates the presence of cement in two cases. The little secondary crystals which are described in the paper might readily be mistaken for cements, of definite or indefinite composition, if they were found only in certain specimens, and then in such proportions as could be accepted. But they occur everywhere, and in all cases with identical appearances, forms, and dimensions; and, moreover, are seen to be collected into crystallites which pervade the whole mass. These are, therefore, usually and indubitably due to the crystallization of gold itself, although the alloying substances sometimes (indium and probably potassium) join up the crystals in question. For the same reason the dark line of the joints, traced as furrows by the etching, are very rarely the empty tracks of cement which has been dissolved away by aqua regia; their formation, which it is easy to follow in all its phases, directly connects them with secondary crystallization. The authors are led to the belief that in the case of ten of their alloys of the gold with about 0"2 per cent, of various impurities, solidification of the whole mass has been directly accomplished at a single time, and that the foreign bodies have remained as solidified solutions, as they were fluid solutions when the alloys were liquid, the impurities being dissociated into their ions in both solid and liquid. Under these conditions it is difficult to invoke, as explaining the mechanical properties of the alloy, the intervention of hypothetical “ cements ” with relatively low fusing points.

in the second part of the paper attention is directed to the fact that gold alloyed with bismuth, thallium, antimony, and aluminium has its structure entirely changed by annealing it in sulphuric acid at about 250°. The large grains of the metal become divided into a multitude of little polyhedral grains. Nothing- remains of the original structure, and the effect closely resembles that which is obtained by annealing steel castings at a bright red heat (800°). It is pointed out that whatever this observation may signify, the trans-