COPPER, SILVEB, ANE GOLD 449 



above 300 the latter chloride also loses its- chlorine and leaves 

 metallic gold. Auric chloride is the usual form in which gold occurs in 

 solutions, and in which its salts are used in the arts and for chemical 

 purposes. It is soluble in water, alcohol, and ether. Light has a reduc- 

 ing action on these solutions, and after a time metallic gold is deposited 

 upon the sides of vessels containing the solution. Hydrogen when 

 nascent, and even in a gaseous form, reduces gold from this solution 

 to a metallic state. The reduction is more conveniently and usually 

 effected by ferrous sulphate, and in general by the action of ferrous 

 salts. 38 



If a solution of potassium hydroxide be added to a solution of auric 

 chloride, a precipitate is first formed, which re-dissolves in an excess of 

 the alkali. On being evaporated under the receiver of an air-pump, 

 this solution yields yellow crystals, which present the same composition 

 as the double salts AuMCl 4 , with the substitution of the chlorine by 

 oxygen that is to say, potassium aurate, AuK0 2 , is formed in crystals 

 containing 3H 2 O. The solution has a distinctly alkaline reaction. 

 Auric oxide, Au 2 O 3 , separates when this alkaline solution is boiled with 

 'an excess of sulphuric acid. But it then still retains some alkali ; how- 

 ever, it may be obtained in a pure state as a brown powder by 

 dissolving in nitric acid and diluting with water. The brown powder 

 decomposes below 250 into gold and oxygen. It is insoluble in water 

 and in many acids, but it dissolves in alkalis, which shows the acid 

 character of this oxide. An hydroxide, Au(OH)^ may be obtained as a 

 brown powder by adding magnesium oxide to a solution of auric chlo- 

 ride and treating the resultant precipitate of magnesiu no aurate with 

 nitric acid. This hydroxide loses water at 100. and gives auric oxide. 34 



53 Stannous chloride as a reducing agent also acts on auric chloride, and gives a red 

 precipitate known as purple of Cassius. This substance, which probably contains a 

 mixture or compound of aurous oxide and tin oxide, is used as a red pigment for china 

 \and glass. Oxalic acid, on heating, reduces metallic gold from its salts, and this property 

 may be taken advantage of for separating it from its solutions. The oxidation which 

 then takes place in the presence, pf water may be expressed by the following equation : 

 5AuClj + 8C 2 H. 2 O4 = 2Au + 0HCl+ 6CO 2 . Nearly all organic substances have a reducing 

 action on gold, and solutions of gold leave a violet stain on the skin. 



Auric chloride, like platinic chloride, is distinguished for its clearly-developed 

 property of forming double salts. These double salts, as a rule, belong to the type 

 AuMCl 4 . The compound of auric Jchloride with hydrochloric acid mentioned above 

 evidently belongs to the same type. The compounds 2KAuCl 4 ,5H 2 O, NaAuCl 4 ,2H 2 O, 

 ANH 4 C1 4 ,H 2 O, Mg'AuCl 4 ) 2 ,2H 2 O, and jthe like are. easily crystallised in well-formed 

 crystals*. Wells, Wheeler, and Penfield "(1892) obtained RbAuCl 4 (reddish yellow) and 

 CsAuCl 4 (golden yellow), and corresponding bromides (dark coloured). AuBr 3 is ex- 

 tremely like the chloride. Auric cyanide is obtained easily in the form of a double salt 

 of potassium, KAu(CN) 4 , by mixing saturated and hot solutions pf potassium cyanide* 

 with auric chloride 1 and then cooling. 



34 If ammonia be 'added to a solution of auric chloride, it forms a yellow precipitate 



