220 Dr. T. Kirke Kose. On certain [Feb. 9, 



of assay, which involve dissolution in nitric acid, found to be much 

 more convenient. When the alloy was digested with nitric acid in 

 the ordinary way, at a moderate temperature, large and variable 

 quantities of silver were retained by the gold. When the alloy was 

 boiled in moderately strong nitric acid for 30 minutes, the gold still 

 retained about 0'2 per 1000 of the silver, and a further variable amount 

 was carried off by the vapour rising from the boiling liquid and lost. 



It was, therefore, necessary to discover some other alloy which 

 would be at once uniform in composition and easy to use as a standard 

 of comparison. 



An examination of the cooling curves of a number of alloys con- 

 taining 92-5 per cent, of silver afforded evidence that the silver- 

 cadmium alloy would prove to be the most suitable one for trial- 

 plates. Unlike the alloys containing gold, tin, or antimony, the 

 cadmium alloy presents no difficulty in connection with the assay by 

 the ordinary methods, and the cooling curve, B fig. 1, bears a close 

 resemblance to that of a pure metal, the fall in temperature during 

 solidification not being great. The completion of solidification, how- 

 ever, is marked by a slight momentary change of direction of the 

 curve, pointing to the freezing of a small quantity of material of the 

 nature of an eutectic alloy, and it seemed possible that this might 

 correspond to a change in the distribution of the silver throughout 

 the mass, resulting in a lack of homogeneity. 



In order to test this, ingots of silver and cadmium of standard 

 fineness were prepared and rolled out. Some difficulties were at first 

 encountered in the melting of the alloy. If pieces of solid cadmium, 

 even when warmed, are added to molten silver, the action is very 

 violent, and varying losses of cadmium by oxidation and volatilisation 

 take place, so that the resulting alloy is seldom of the exact composi- 

 tion required. The method eventually adopted consists in pouring 

 molten silver, heated only slightly above its melting point, into a 

 large crucible containing melted cadmium at a black heat and covered 

 with charcoal. The crucible is then placed in the fire, raised to a 

 bright red heat as quickly as possible, and the alloy well stirred and 

 poured into a closed mould. Under these conditions, when operating 

 with 4 or 5 kilos, of material, the mixing of the metals takes place 

 with comparative quietness, and the loss of cadmium is reduced to 

 about 0*15 per cent, by weight of the alloy. In order to prepare a 

 standard trial-plate, which consists of 925 parts of silver and 75 parts 

 of base metal, it is accordingly necessary to melt 76*5 parts of 

 cadmium and to add 925 parts of silver to it. A large part of the 

 loss appears to take place on the addition of the silver to the cadmium, 

 dense fumes of oxide of cadmium being given off, but on merely 

 melting and re-casting a standard ingot of 5 kilos., the proportion of 

 silver present rises from 92-50 per cent, to about 92-58 per cent. 



