676 GOLD. 



hydrogen gas is conveyed into a boiling solution of the perchlo- 

 ride of gold. It is dark brown, almost black. 



Peroxide of gold, Auric oxide, Au 2 O 3 , 2786 or 223.21. 

 This oxide has many of the properties of an acid. It is obtained 

 by digesting magnesia in a solution of perchloride of gold, 

 when an insoluble compound of auric oxide and magnesia is 

 formed, which is collected upon a filter and well washed. The 

 compound is afterwards digested in nitric acid, which dissolves 

 the magnesia, with traces of auric oxide, but leaves the greater 

 part of the latter undissolved. It is left in the state of a reddish 

 yellow hydrate, which dried in air becomes chestnut brown. 

 When precipitated by an alkali, auric oxide carries down a por- 

 tion of the latter, of which it may be deprived by nitric acid. 

 Dried at 212 it abandons its water, becomes black, and is in 

 part reduced. When exposed to light, particularly to the direct 

 rays of the sun, its reduction is very rapid. It is decomposed 

 by an incipient red heat. Hydrochloric acid is the only acid 

 which dissolves and retains this oxide, and then perchloride 

 of gold is formed. It is dissolved by concentrated nitric and 

 sulphuric acid, but precipitated from these solutions by water. 

 The affinity of this oxide for alkaline oxides is so great, on the 

 contrary, that when boiled in a solution of chloride of potas- 

 sium, auric oxide is dissolved, and the liquid becomes alkaline ; 

 perchloride of gold with chloride of potassium, and aurate of 

 potash, or a compound of auric oxide and potash, are formed. 

 The compounds of auric oxide with the alkalies and alkaline 

 oxides are nearly colourless, and are not decomposed by water. 

 They appear to be of two different degrees of saturation, aur- 

 ates which are soluble, and superaurates which are insoluble. 

 The only one of these compounds which has been studied in 

 some degree is the aurate of ammonia, or fulminating gold as it 

 is named from its violently explosive character. 



Aurate of ammonia. When the solution of gold is precipi- 

 tated by a small quantity of ammonia, a powder of a deep yel- 

 low is obtained, which is a compound of aurate of ammonia 

 with a portion of perchloride of gold. This compound is ex- 

 ploded by heat, but the detonation is not strong. But when 

 the solution of gold is treated with an excess of ammonia, and 

 the precipitate well washed by ebullition in a solution of ammo- 

 nia, or better in water containing potash, the fulminating gold 

 has a yel) jwish brown colour with a tinge of purple. When dry, 



