Chap. 35.] on Gold explained. 233 



acid has no other effect than that of furnilhing Oxygen, 

 fmce the fame effect follows when the neceflary quan- 

 tity of oxygen is previoufly added either to the gold or 

 the muriatic acid. Gold, precipitated from aqua regia 

 by alkalis, and thus reduced to the calciform (late, is 

 foluble even in the vitriolic and nitrous acids. 



The addition of water to the vitriolic acid, enables it 

 to difiblve iron exactly on the fame principle that the 

 addition of nitrous acid to the muriatic, enables the 

 latter to diffolve gold. The concentrated vitriolic 

 acid has no action on iron without the -afliftance of 

 heat ; but by a proper addition of water the procefs 

 goes on, in the ordinary temperature of the atmofphere, 

 with confiderable rapidity. The water furnifhes oxy- 

 gen to the iron, and its other component part, hydro- 

 gen, is fet at liberty j and that the nitrous acid, in the 

 aqua regia, anfwers the purpofe of furnifhing oxygen 

 to the gold, is proved by the difengagement of nitrous 

 gas. 



The folution of gold in aqua regia, when fifft 

 made, is always yellow, considerably cauftic, corrodes 

 animal matters, and tinges them of a deep purple 

 colour. When applied to the furface of marble, 

 it tinges it of a violet colour. This colour is pro- 

 duced by a precipitation of the gold, in confequence 

 of the fuperior attraction of the calcareous earth for 

 the acid. 



The folution of gold may be made, by cautious 

 evaporation, to afford cryftals of a beautiful tonaz or 

 yellow colour. Gold may be in fome meafure vola- 

 tilized by repeatedly diftilling it with aqua regia ; fome 

 of the gold rifes with the acid into the neck of the 

 retort in the form of long (lender brown cryftals. 



Gold is precipitated from its folution by a great 



variety 



