1890.] The Liquation of Gold and Platinum Alloys. 181 



about a true alloy. It appeared to me that alloys of gold and 

 platinum would well repay examination. They have been generally 

 considered to be uniform in composition, but certain results which I 

 obtained in the course of their treatment led me to suspect that they 

 would give interesting results, and the following experiments were 

 therefore undertaken. 



The metal platinum frequently occurs in the gold and silver bullion 

 which has to be treated by the ordinary methods of refining, and its 

 presence occasions no small amount of trouble to the refiner. 



It is well known that there are two methods of refining, both of 

 which involve alloying one part of gold with (about) three parts by 

 weight of silver, and treating the mass directly — 



(a.) With nitric acid, 

 (5.) With sulphuric acid. 



The final result from either method, if properly conducted, is fine 

 gold and fine silver — that is to say, if the alloy so treated is com- 

 posed of gold and silver only (a little copper present making no 

 difference). 



In the case of platinum being present in the gold or the silver, if it 

 is refined by the nitric acid process, the platinum, when existing in 

 small proportions, is eliminated with the silver, becoming dissolved 

 up with it, leaving the gold free, and the platinum so dissolved can 

 afterwards be readily separated from the silver, but upon the large 

 scale refining by means of nitric acid is far too costly ; practically, 

 therefore, this has to be replaced by the sulphuric acid process. 



In an alloy of gold and silver, containing a small proportion of 

 platinum, nearly all the silver is dissolved by the sulphuric acid, 

 leaving the platinum associated with the gold. 



In order to simplify matters for further treatment, this partially 

 refined gold holding the platinum is melted and assayed, to deter- 

 mine the amount of platinum and gold it contains ; it is the platinum- 

 gold alloys so obtained that I desire to bring under notice. 



It has been found in practice that the ordinary method of assaying 

 a small portion cut from one end of a bar or ingot of such metal 

 does not indicate the actual percentage of gold and of platinum exist- 

 ing in the entire mass, and it is therefore evident that the platinum 

 has been redistributed by liquation during the cooling and solidifica- 

 tion of the mass. 



Having been struck by the experiments made by Professor Roberts- 

 Austen, as detailed in the paper to whieh reference has already been 

 made, I cast some gold containing platinum into a special iron mould 

 3 inches in diameter, and cut the spheres of metal so obtained in two 

 halves. I may mention that I had to cast these spheres many times 

 over in order to obtain a solid casting, so great was the shrinkage. 



