On Certain Properties of the Alloys of the Gold-Copper Series. 105 



sion. I succeeded in 1895 in obtaining some evidence as to the separa- 

 tion of gold from its solution in metallic lead by electrolysis through a 

 glass septum.* This is, however, only indirectly connected with the 

 electrolysis of alloys.] 



" On Certain Properties of the Alloys of the Gold-Copper Series." 

 By Professor Sir W. KOBERTS-AUSTEN, K.C.B., F.E.S., and 

 T. KIRKE PiOSE, D.Sc. Eeceived and read May 10, 1900. 



[PLATE 1.] 



Notwithstanding the extraordinary importance from a technical 

 point of view of the members of this series, which constitute the gold 

 coinages of the world, singularly little is known respecting either their 

 molecular constitution or even their physical constants. Both the 

 authors of this paper possess unusual facilities for studying them, and 

 they felt that time should not be lost in beginning a systematic exami- 

 nation of the series. The other alloys used for coinage have, on the 

 other hand, not been so neglected. Many years ago one of us,t in 

 submitting his first paper to this Society, gave a curve representing 

 the freezing points of the members of the silver-copper series. This 

 curve, corrected in accordance with more recent work and interpreted 

 in a modern way, proved to be one with two branches meeting at a 

 point where the eutectic alloy of the two metals occurs. The presence of 

 the eutectic has also been since readily detected in standard silver and 

 in several other members of. the series, and possesses a melting point of 

 778. As is well known, different portions of a mass of any of the 

 solidified alloys of the silver-copper series, except the eutectic alloy, 

 exhibit divergences in composition which usually amount to about two 

 or three parts in a thousand. 



The gold-copper series, on the other hand, has long enjoyed a 

 reputation for homogeneity, and it was supposed that the variations in 

 the composition either of the alloy which contains 916 - 66 parts of gold 

 in 1000, and is used for the coinage of the Empire, or of the alloy 

 which contains 900 parts of gold in 1000, and is one adopted by the 

 Latin Union and in the United States of America, need not exhibit 

 greater divergences than O'l part in 1000. It was, moreover, believed 

 that such a divergence was not the result of any systematic molecular 

 grouping. This view was shaken by one of usj in 1895, when 

 evidence was obtained by chemical analysis that in the case of a gold- 



* Third Keport to the Alloys Research Committee, ' Proc.Inst. Mech. Engineers, 

 1895, p. 240. 



t Roberts- Austen, ' Roy. Soc. Proc.,' vol. 23 (1874), p. 481. 

 Rose, ' Chem. Soc. Journ.,' vol. 67, 1895, p. 552. 



