138 TELLURIUM ORE. 



ammonium. The precipitate obtained by this last re- 

 agent is insoluble when cold in an excess of sulphide of 

 ammonium: it dissolves, on the contrary, when heated 

 in this liquid. By cooling the sulphide is precipita- 

 ted ; but it is white in this case, and is probably a 

 hydrate. 



74. TELLURIUM OEE. 



The Transylvania powdered ore contains graphic 

 and foliated tellurium-ore; i.e., the tellurides of gold, 

 silver, lead, and sulphide of tellurium, mixed with 

 various other minerals. 



In order to remove a great portion of the gangue, 

 the finely-powdered ore is mixed with dilute hydro- 

 chloric acid, with which it is left in contact until no 

 farther disengagement of carbonic acid takes place, 

 the whole being frequently agitated by stirring. It is 

 then washed and dried. Various methods can be 

 used to extract the tellurium and to recover at the 

 same time the noble metals. 



I. The ore is dissolved in nitro-hydrochloric acid, 

 with the precaution, however, that the nitric acid is 

 only gradually added, and in such quantities that it 

 may all be decomposed. When the mass has become 

 completely white, and all the nitric acid has been ex- 

 pelled by heat, some sulphuric and tartaric acids are 

 added, the former to insure the complete precipitation 

 of the lead and the decomposition of the tellurite of 

 lead, the latter to prevent the precipitation of tellurous 

 acid; after this, about twice its bulk is added to the 

 mass. When completely cold, the solution is filtered 

 off and the residue washed. The latter consists of 

 quartz, heavy spar, sulphate of lead, and a small quan- 

 tity of chloride of silver, which may be extracted by 

 amii.onia. 



