ON GOLD-ALUMINIUM ALLOYS. 215 



a more or less golden tinge, and under the microscope show smears of gold, or a 

 pattern of gold, on white or purple ; and in cases where the polishing leaves a pitted 

 surface, the pits are full of gold. On the other hand, the unpolished alloys are white 

 if they contain more than 3 per cent, by weight of aluminium, except in the neigh- 

 bourhood of H, where they are purple. This constant appearance of free gold on the 

 polished surfaces troubled us a good deal at first, but we finally satisfied ourselves 

 that most of it had been set free by the superficial oxidation of the alloys during 

 polishing, and that after the oxidation of the aluminium the gold was smeared over 

 the harder surfaces and rubbed into the pits. This, whether it be due to oxidation 

 or not, can be to a large extent avoided by finishing the polishing on emery kept wet 

 with benzene. The alloys are then almost free from gold smears, and we believe 

 that in the solid unpolished alloys containing more than 20 atoms of aluminium, that 

 is, after the point B, there is no free gold. This is an important point, for if free 

 gold occurs to the right of B, the steps in the process of solidification become difficult 

 to understand. One of the uses of the etching is to remove these smears of gold. 



The types of pattern visible on the etched surface depend on the position of the 

 freezing point in the curve. It is hardly too much to say that, given the freezing 

 point-curve of any pair of metals, one can predict the microscopical structure of the 

 alloys they form. 



For alloys whose freezing point is near a summit of the curve, that is, near A, E, 

 H, and probably J, the whole surface of the section is filled with one substance, 

 although it is sometimes possible to detect fine boundary lines marking out the 

 separate crystals. These lines are most often seen at the angles where three crystals 

 meet, in which case (figs. 18 and 21) the boundary line consists of three branches 

 meeting in a point. 



When, by the addition of either metal, we leave a summit of the curve, the lines 

 between the polygonal sections of the crystals become distinct, so that the pattern is 

 that of a tesselated pavement, the unit being an irregular polygon, generally without 

 re-entrant angles, often approximating to a regular hexagon, and often with some- 

 what rounded angles, so that it may be called a blob. It does not seem necessary to 

 attribute the hexagonal shapes to any peculiarity of crystalline structure, but rather 

 to the limitations of space in which the closely packed crystals have formed.* As 

 we go further down hill along the curve, the spaces between the polygons widen and 

 are seen to be full of a substance different from that of the polygons themselves. As 

 we still go down-hill the interpolygonal matter becomes a continuous network, and 

 the isolated polygons or blobs arrange themselves into patterns. Sometimes, as with 

 16 "9 atoms of aluminium, the pattern is mainly one of rectangular crosses, but more 

 often the blobs are in rows with other rows branching from them, the individual 



* Mr. J. E. STEAD draws attention to the non-crystalline character of the shape of these polygons in his 

 valuable paper on " The Crystalline Structure of Iron and Steel," in the ' Journal of the Iron and Steel 

 Institute,' 1898. 



