ON GOLD-ALUMINIUM ALLOYS. 223 



The bodies D and E are so similar under a high power, that we found it difficult 

 to believe them to be chemically unlike each other, but the form of the curve, and 

 the relative positions of the two bodies in the sections examined, are both best 

 explained by the hypothesis of their not being the same substance. The E body, 

 which is most certainly Au 2 Al, is in equilibrium with the liquid along the curve 

 ED, while at D there is a triple point. This point corresponds to a temperature 

 and concentration at which both solid bodies can exist in equilibrium with the liquid. 

 At points between E and D we therefore have primary crystals of E immersed in 

 white mother-substance. The door-panel moulding, distinctive of the E body, and 

 described later, is found on the primary crystals as far back as 28 '3 atoms. The 

 mother-substance along DE is very white and uniform, and does not resolve into a 

 eutectic mixture, but under the 2 millims. immersion it shows a tendency to break up 

 into polygons, with thin lines between them. This can with care be detected in all 

 sections, from 27 to 31 atoms of aluminium, and is just what we should expect to see 

 if D, the intersection of the two branches CD and DE, lay a very little on one side 

 of the summit of the branch CD. 



While we are confident that the E body is Au 2 Al, we cannot feel the same con- 

 fidence as to the formula of the body which would lie pure at the summit of the 

 branch CD. The formula Au 8 Alj would put the summit at 27 '3 atoms of aluminium. 

 This would fit in very well with the curve ; and our alloy (fig. 14), which professes 

 to contain 27 '2 atoms, has, as the photograph shows, very little mother-substance 

 between the polygons ; that is, it is nearly a pure body. On the other hand, the 

 formula Au 9 Al 2 puts the summit of CD at 28 '6 atoms. Alloys of this composition 

 can be seen in the photograph (fig. 16) to have in them a small quantity of primary 

 crystals of E, so that if Au s AL is the formula, the summit must lie a little under- 

 neath the branch DE, and be therefore unrealisable. 



Before leaving the branch DE, the very rounded shape of the blobs of the primary 

 crystals of E should be noticed. Their shape is just what one would expect to see if 

 an emulsion of two conjugate liquids were suddenly solidified. We have been some- 

 times strongly tempted to think that E did separate in liquid drops along this 

 branch. The same effect is seen, but not so strongly, along EF, and indeed in almost 

 all the primary crystals, but such a supposition would leave unexplained the larger 

 regular patterns in which the blobs arrange themselves, and we are disposed to think 

 that, as we have already stated, the blobs are filled out crystal skeletons. 



The alloy Au 2 Al, which is pure at E, shows when examined with a high power, a 

 great deal of detail. Many of the polygons are ruled with fine lines (fig. 19), the 

 direction of ruling being often different in different polygons. These patches of 

 course present the rotation effect. In many places it shows a beautiful crystalline 

 structure with a pattern resembling the moulding of the panels of a door, and not 

 unlike the well-known appearance of crystalline bismuth. We give a photograph of 

 this (fig. 20) taken with a magnification of 450 diameters. Practically no eutectic is 



