MANUAL OF NATURAL HISTORY. 597 



A rock supposed to contain gold is tested 

 by a portion being powdered and washed in 

 a shallow iron pan, upon which most of the 

 gold will subside, this process is repeated 

 until the principal impurities are carried off, 

 when mercury is added, which forms an 

 amalgam with the gold ; this is strained, to 

 carry off any excess of quicksilver, and the 

 remainder is expelled by the application of 

 heat, leaving the pure gold. Gold is soluble 

 only in Nitro-hydrochloric acid (aqua regia), 

 from which it may be precipitated pure by 

 sulphurous acid gas, and by a solution of 

 sulphate of iron (green vitriol) ; as a Sul- 

 phuret by sulphuretted-hydrogen gas ; or as 

 Purple of Cassius, by solution of protochlo- 

 ride of tin. Symb. Au. 



2. FAMILY. Platinides. Minerals consisting of, 



or containing principally, Platinum. Almost 

 always combined with other precious metals; 

 often in gold alluvial or diluvial washings ; 

 H. 4~4'5 ; sp. gr. 173, or 21'5 when pure; 

 resembling silver, but less lustrous ; soluble 

 in nitro-hydrochloric acid, from which a so- 

 lution of chloride of ammonium (sal-ammo- 

 niac) throws down a yellow, insoluble com- 

 pound. Symb. Pt. 



3. FAMILY. Palladiides. Minerals containing Pal- 



ladium. This metal occurs in Brazilian pla- 

 tinum-ore, as small flattened grains, also in 

 auriferous sand; H. 4*5-5; sp. gr. ITS 

 Symb. Pd. 



