1902.] Properties of the Alloys of the Gold-Silver Series. 



163 



There is, of course, no eutectic alloy observable in any member of 

 the series. 



The alloys all consist of large grains, but these are built up of 

 smaller grains, so that the ultimate structure is exceedingly minute. 

 When magnified 1500 diameters the grains appear as small irregular 

 crystals of the cubic system (see fig. 2, PI. 3). In order to develop any 

 segregation that might take place, an ingot of the standard alloy 

 containing 91 '66 per cent, of gold by weight was heated for 2 months 

 in one of the annealing furnaces at the Royal Mint, the temperature 

 of which was kept at about 700° by day, but fell to about 100° at 

 night. The maximum temperature attained was over 300° below the 

 fusing point of the alloy, and the sharpness of the angles of the 

 specimen had suffered no change. After this treatment it was found 

 that the grains had increased in size, and the crystals forming them 

 had become well developed, as shown in fig. 3 (PI. 3), in which the struc- 

 ture is magnified 1500 diameters. No true segregation, however, could 

 be detected even in this ingot, either by analysis or by the microscope, 

 and plates prepared by rolling out ingots containing 916 - 6 parts by 

 weight of gold, and 83*3 parts of silver, were found on analysis to be 

 uniform in composition. 



The ancient trial plates, according to the analysis made by one of 

 us,* consist of a triple alloy of gold, silver, and copper. The earliest 

 one in existence was made in 1527, the year following the first intro- 

 duction of the standard 916 6. This plate contained only 0*62 per 

 cent, of copper, and was probably intended to consist of gold and 

 silver only. All subsequent plates, however, down to that made in 

 1829, contained much larger amounts of copper. In 1873 it was 

 determined to omit the silver and to use only copper as the alloying- 

 metal, and thus to preserve identity of composition between the trial 

 plate made in that year and the coinage. In view, however, of the 

 importance of obtaining homogeneous trial plates and of the ease with 

 which the exact quantity of copper required to make the assay pieces 

 identical in composition can be added to the pieces of the trial plate 

 during the course of the assays, it is preferable to use only silver as 

 the alloying metal in the manufacture of the trial plates. 



Such an alloy has accordingly been used at the Royal Mint since 

 the beginning of the present year instead of fine gold for checks in the 

 assay of standard bars and coins. In view of the minute accuracy 

 with which the operations of coinage have to be conducted, this is a 

 matter of much importance. By this method any errors are avoided 

 which might be caused by accidental variations in weights occurring 

 after the trial plates have been made. 



* Koberts-Austen, ' Chem. Soc. Jonrn.,' 1874, p. 197. 



