224 MESSRS. C. T. HEYCOCK AND F. H. NEVILLK 



visible between the crystal aggregates, but in some parts of the section the polygons 

 are marked out by very fine lines ; one polished section showed this before etching. 

 The alloy itself is a somewhat brittle white substance. Etching with bromine or 

 aqua regia develops large patches of various shades of grey, some very silvery : 

 these are well seen with a hand lens. Oblique illumination, with a power of 50 

 diameters, shows groups of silvery spots on a dark ground. These spots disappear on 

 rotation of the stage, and others become visible. The large patches also change 

 from light to dark in the same way. As far as one can see, the whole surface shows 

 this rotation effect, and consists of groups of polygons of the same substance in 

 different orientations. 



The alloys between E and G are a mixture in varying proportions of the above 

 described Au 2 Al and of the body named X. This latter, when unetched, is of a fine 

 ivory white, but etching attacks it more rapidly than is the case with the E body, 

 and turns it grey or brown. All the alloys between E and G are, when unetched, a 

 mixture of pure white X and of E, which is often covered with gold. The effect of 

 etching is to remove this gold and to show the pure white of Au 2 Al, while the 

 X body is darkened and rapidly eaten away. The alloy with 35 '1 atoms (fig. 21) 

 shows the smooth and the rough kinds of E and the door-panel moulding, and there 

 is only a little mother-substance, the structure being that of a nearly pure body. 

 The alloy with 36 '6 atoms of aluminium, after etching with aqua regia, is shown, by 

 a power of 50 diameters, to consist of silvery blobs almost isolated from one another. 

 With a power of 500 this is a magnificent section. The white silvery spots of 

 Au.,Al, each uniformly pitted, are surrounded by a eutectic consisting of a minute 

 pattern of the .same white, intimately mixed with the X body, which is grey or 

 brown. With 38 atoms etched in the same way there are about equal amounts of 

 the pitted form of E, and of the eutectic made up of E and X. The alloy with 

 38 "9 atoms (fig. 22) shows a very good eutectic between the larger crystals of E. 

 That with 39 '9 atoms shows lines of silver dots of E, very slender, but beautifully 

 rectangular in arrangement. The eutectic, which fills nine-tenths of the whole area, 

 is resolved by a power of 500 into its two components. We are now very close to 

 the eutectic point F, but still have a slight excess of gold. A slowly-cooled alloy, 

 containing 40 atoms of aluminium, is the large scale but very uniform eutectic given 

 in fig. 23. But one point of this alloy showed a primary crystal of Au 2 Al that, 

 magnified to the scale of the photograph, would be 2 inches across ; this was the only 

 primary crystal in the section. We mention its size to emphasise the relatively fine 

 grain of the eutectic. 



With 407 atoms the positions of the two ingredients are reversed ; instead of the 

 scanty rows of white spots of the alloy at 39 '9 atoms, we now have a few very 

 straight rows of spots of grey X immersed in the same eutectic. We have thus 

 crossed over to the other side of the eutectic point F, and the X body is crystallising 

 first. The alloy with 427 atoms, like all those between F and G, polishes well, 



