On Certain Properties of the Alloys of the Gold-Copper Series. 109 



It has been shown by Osmond,* moreover, that silver and copper 

 are each capable of holding a small percentage of the other in solid 

 solution, but that if both metals are present in considerable amounts, 

 the two solidified solutions exist side by side. It is evident therefore 

 that they form an interrupted series of " mixed crystals," and that the 

 substance first solidified in cooling a solution of one metal in the other 

 is not a pure metal, but an isomorphous mixture of the two metals 

 containing only a small percentage of one of them. This conclusion 

 agrees well with the general shape of the curve of fusibility of the 

 silver-copper series, and the still greater concavity of the curve of 

 fusibility of the gold-copper series suggested that a similar condition 

 of things is here met with, but that the gap in the series of mixed 

 crystals is much smaller, and that the mutual solubility of these two 

 metals is greater. 



Microscopic examination of the alloys of gold and copper affords 

 evidence that this is really the case, but appears to point to the con- 

 clusion that more copper can be dissolved in gold than gold in copper. 

 Alloys containing only a small percentage of copper consist of large 

 crystals similar in shape to those seen in pure gold, and showing no 

 signs of cement between them. They differ from those of pure gold 

 in their colour, which is reddish or reddish-brown, after treatment 

 with nitrohydrochloric acid. When magnified 1580 diameters these 

 crystals show a minutely granular structure which resembles that of 

 pure gold, and affords no evidence of separation into two constituents. 

 Even in standard gold containing only 9T6 per cent, of gold the 

 structure is nearly the same, and is not unlike that of the ground 

 mass of standard silver containing 92 -5 per cent, and 7*5 per cent, of 

 copper prepared in a similar way. On the other hand, the alloys con- 

 taining less gold than the eutectic show crystals of copper set in a 

 matrix which consists apparently of the eutectic. 



The following examples of photomicrographs of the series are shown 

 in Plate 1 : 



Fig. 1, Plate 1, represents the characteristic surface of a small ingot 

 of standard gold. The structure was not developed by etching, and 

 the magnification is only 4'5 diameters. 



Fig. 2 is a polished section of standard gold etched by immersion 

 for about 15 seconds in a boiling mixture of equal parts of nitric and 

 hydrochloric acid. The magnification is, as in the case of No. 1, 4*5 

 diameters, and the structure consists of sections cut in various direc- 

 tions by a plane passing through the crystals, of which the mass is 

 composed. Fig. 3 is the eutectic of the gold-copper series ; it contains 

 80 per cent, of gold and 20 per cent, of copper etched as in the case of 

 the alloy shown in fig. 2 ; the magnification is, however, 1580 diame- 



* ' Bull, de la Soc. d'Encouragement,' 5th Series, vol. 2 (1897), p. 837. 



