196 METALS AND THEIR COMBINATIONS. 



Sufficiently evaporated, the solution yields crystals of the 

 composition SnCl 2 .2H 2 O. Stannous chloride is a strong deoxi- 

 dizing agent, frequently used as' a reagent for mercury and 

 gold, which metals are precipitated from their solutions in the 

 metallic state. It is also used in calico-printing. 



Stannic chloride, SnCl 4 (Perchloride of tin). Stannous chloride 

 may be converted into stannic chloride either by passing chlo- 

 rine through its solution or by heating with hydrochloric and 

 nitric acids. 



Analytical reactions. 



1. Add hydrosulphuric acid to solution of a stannous salt: 

 brown stannous sulphide is precipitated : 



SnCl 2 + H 2 S = 2HC1 + SnS. 



The precipitate is soluble in ammonium sulphide. 



2. Add hydrosulphuric acid to solution of a stannic salt: 

 yellow stannic sulphide is precipitated: 



SnCl 4 + 2H 2 S = 4HC1 + SnS 2 . 



The precipitate is soluble in ammonium sulphide. 



3. Sodium or potassium hydrate added to a stannous salt, pro- 

 duce a white precipitate of stannous hydrate, Sn2HO. The 

 same reagents added to a stannic salt produce white stannic 

 acid, H 2 Sn0 3 . Both precipitates are soluble in excess of the 

 alkali. 



Gold, Au = 196.2 (Auruni). Gold occurs in nature in the free 

 state, and is separated from adhering sand and rock by a me- 

 chanical process of washing, in which advantage is taken of the 

 high specific gravity of gold. 



Pure gold is too soft for general use, and is therefore alloyed 

 with various proportions of silver and copper. American coin 

 is an alloy of 90 parts of gold and 10 parts of copper; jeweller's- 

 gold contains generally 75 per cent. (18 carat) of gold, the other 

 25 per cent, being copper and silver; the varying proportions 

 are well indicated by the color. 



Gold is not affected by either hydrochloric, nitric, or sul- 



