220 MESSRS. C. T. HEYCOCK AND F. H. NEVILLE 



conchoidal fracture. Thus near B, on both sides of it, we find the structure 

 characteristic of an almost pure substance. 



The next alloy with 21 "6 atoms shows a pattern that varies a good deal in different 

 parts, but the most usual is that of armies of very minute dots and short lines of white 

 in a ground which, after etching, is a golden brown. There are a few larger spots of 

 white, so that we may have now just passed the eutectic angle C. In the middle of 

 some of the brown spaces, between the bars and dots of white, a power of 1200 shows 

 patches of a network of white lines, similar to that on the golden mother-substance of 

 the 2G'l atom alloy to be referred to later ; this is probably the real eutectic 

 structure (fig. 9). 



These features in the alloys, taken together with the shape of the curve between 

 A and C, and especially the horizontal line of second freezing points at the tem- 

 perature B, make it extremely probable that the solid alloy containing 20 atoms of 

 aluminium is a homogeneous chemical compound with the formula Au + Al. With 

 rather more gold the solid alloy must consist of primary crystals of Au 4 Al in a 

 eutectic of this body and gold, the eutectic angle being probably near 19 atoms. 

 With more gold we have, as at 18*1 atoms, primary crystals of gold immersed in a 

 eutectic mixture of gold and Au 4 Al. 



With more than 20 atoms of aluminium we are on the new equilibrium curve BC, 

 along which Au 4 Al should crystallise first, and be found in the solid alloy embedded 

 in a eutectic containing also the new white body D. Unfortunately we have not 

 enough alloys on this branch, but the 21'1 atom alloy (figs. 7 and 8) unmistakably 

 shows this white substance. As the rod of extract at 19'-8 atoms shows, both before 

 and after polishing, the body Au 4 Al is itself a white substance, but it is very readily 

 attacked by etching reagents, and therefore under the microscope it appears brown 

 or purple from a film of gold. 



As the alloy Au 4 Al would contain only a little more than 3 per cent, by weight of 

 aluminium, it is obvious that our analyses, though very consistent with each other, 

 leave the exact atomic percentage at B a little indefinite ; the method of plotting in 

 atomic percentages may be said to magnify the scale of the curve here, on account of 

 the disproportion between the atomic weights of the two metals. But we think it 

 justifiable to use the law of multiple proportions in selecting the formula Au^Al as 

 that of the compound which is pure near B. 



We have three sections of alloy containing 22 '3 atoms of aluminium ; they are all 

 similar, and have a curious pattern. With a low power of 50 or 100 one sees dark 

 polygons outlined not by lines but by small somewhat oval spots of white, which 

 punctuate, as it were, the margin of the polygons. This is best shown in a section of 

 a part of the alloy which ran out of the pipette after removal from the crucible. 

 This must have been the most fusible part of the extract, and was therefore probably 

 very near the composition of the eutectic C. With a higher power and the 2 millims. 

 immersion one sees that these white spots have all the character of the D body, they 



