110 Sir \V. !!..l..Tt<-AnsicM an.l Dr. T. Kirke Rose. 



ten, whirh reveals the banded structure characteristic of a eutectic 

 alloy. 



Fig. 4 is standard gold etched as before and magnified 

 diameters. 



Fig. 5 is a section, etched as before, of an alloy containing 27 per 

 i cut. of gold, and 73 per cent, of copper. In it the presence of two 

 distinct constituents can be seen. The darker portion, which has 

 been readily attacked by the acid, is copper, and the lighter is mainly 

 the eutectic. This fact is proved by fig. 6, which is a very high nia.u'ni- 

 tiratinn (6300 diameters) of the lighter portion of fig. 5, and this on 

 close examination reveals the presence of the laminated or banded 

 eutectic. 



Another resemblance between the series of gold-copper and silver- 

 copper alloys is to be found in their relative tensile strengths. In 

 both cases the eutectic alloys are extremely brittle, and have a lower 

 tenacity than the other members of the series. In the case of gold, 

 one of us* has shown that the tenacity of an unworkedcast bar of pure 

 gold 7*5 mm. wide and 5'2 mm. thick is 7 tons per square inch, the 

 metal elongating 30'8 per cent, before rupture. Both tenacity and 

 extensibility are greatly increased by the first additions of copper, the 

 tenacity rising in the case of standard gold which contains 8'3 per 

 cent, of copper to more than twice that of pure gold. Under similar 

 conditions, however, we have found that the eutectic alloy of gold and 

 copper has a tensile strength of only 7 '87 tons per square inch, with 

 an elongation of only 3*3 per cent. It is in fact about as brittle as 

 pure gold alloyed with 0'24 per cent, of lead, which has an elongation 

 of 4*9 per cent. We also determined the extensibility of the eutectic 

 alloy of silver and copper to be only 2 '2 per cent., and its tensile 

 strength 29'1 tons per square inch. These are the first cases observed 

 in which eutectic alloys appear to show less tenacity and extensibility 

 than the other members of the series to which they belong. The 

 eutectics of lead and tin, of copper and tin, and of iron and carbon 

 are in each case the strongest alloys of the series, and are not at all 

 brittle. The eutectic of the copper-zinc series is more extensible than 

 any other member of its series, while its tenacity is considerable. The 

 gold-copper and silver-copper alloys differ therefore from other alloys, 

 which appear to be brittle and of low tensile strength only if they 

 have passed through a pasty stage in solidifying, and possess two 

 freezing points, the lower of which is that of the eutectic. 



It is clear, from the results given above, that gold and copper cannot 

 be expected to form a series of alloys of uniform composition, but will 

 show evidence of liquation similar to that exhibited by silver and 

 copper, though in a less degree. Much evidence on this point was 

 obtained in the course of the preparation of the standard gold trial 

 Koberts- A usten, ' Phil. Iran*.,' A, vol. 179 <1888), p. 339. 



