Gold- Aluminium Alloys. 



21 



^ilniost wholly composed of gold, will not begin to solidify until this 

 comparatively low temperature of 527° C. is reached."^ 



Each of these eutectic points gives rise to a horizontal row of second 

 freezing points in the curve, and the alloys containing more than 

 and less than 60 atomic per cents, of alumininm have three distinct 

 freezing points corresponding to the successive formation of three solid 

 bodies. 



The four compounds, Au^Al, AujAlo, AuoAl, and the hypothetical 

 AuAl, are pure white substances. AuAl^, as is well known from the 

 work of Sir W. Roberts- Austen, is a magnificent purple body. 



The latter part of the paper gives the result of a microscopic exami- 

 nation of polished and etched sections of alloys taken from various 

 parts of the curve. All the bodies referred to above could be dis- 

 tinguished under the microscope. The photomicrographs accompany- 

 ing the paper show that the structure of the solid alloy is everywhere 

 in strict harmony with the indications of the freezing point cui've. 

 Speaking generally, we may say that the patterns observed in the 

 photographs repeat themselves at corresponding points of each branch 

 of the ciu:ve. For example, near the summit of the branch corre- 

 sponding to the pure alloy, AuoAl, the photograph shows us more or 

 less hexagonal polygons of this substance almost entirely filling the 

 field, and only separated from each other by very fine lines of im- 

 purity. If we take a section of an alloy a little way below the summit, 

 we see the polygons of AuoAl surrounded by a ribbon-like network of 

 mother substance. Still further down, the crystals of AuoAl are 

 scanty, and arranged in such regular patterns, generally in lines at 

 right angles to each other, as to render it certain that they crystallised 

 freely while surrounded by liquid. Finally, at the bottom of the 

 branch, that is at the eutectic point, the large crystals of AU2AI are 

 absent, and the whole field is full of the mother substance, which is some- 

 times but, as we explain in the paper, not always a eutectic mixture. 



If, leaving the eutectic point, we ascend the next branch, these phe- 

 nomena repeat themselves, but the primary crystallisation (that is the 

 matter which solidified first) is now of a diiferent substance. 



Some of the photographs of alloys very rich in aluminium were 

 taken by the Rontgen rays, and an enlargement made from the nega- 

 tive. The contrast between the Rontgen ray photograph and the 

 surface photograph of the same alloy shows what a much better picture 

 of the structure of the alloy is given by the Rontgen rays. 



* The rapid depression in the freezing point of gold, due to the presence of 

 small quantities of aluminium, and the great rise in the freezing point as the 

 composition corresponding to the compound AuAL is approached, have been 

 already discovered by Sir William Eoberts-Austen. 



