450 PRINCIPLES OF CHEMISTRY 



The starting-point of the compounds of the type AuX 35 is gold 

 monochloride or aurous chloride, AuCl, which is formed, as mentioned 

 above, by heating auric chloride at 185. A'urous chloride forms a 

 yellowish -white powder ; this, when heated with water, is decomposed 

 into metallic gold and auric chloride, which passes into solution * 

 3 AuCl = AuCl 3 -f 2 Au. This decomposition is accelerated by the action 

 of light. Hence it is obvious that the compounds corresponding with 

 aurous oxide are comparatively unstable. But this only refers to the 

 simple compounds AuX ; some of the complex compounds, on the 

 contrary, form the most stable, compounds of gold. Such, for ex- 

 ample, is the cyanide of gold and potassium, AuK(CN) 2 . It is formed, 

 for instance, when finely-divided gold dissolves in the presence of 

 air in a solution of potassium cyanide : 4KCN + 2 Au + H 2 O + O 

 = 2KAu(CN) 2 -f 2KHO (this reaction also proceeds with solid pieces 

 of gold, although very slowly). The same compound is formed in 

 solution when many compounds of gold are mixed with potassium 

 cyanide, because if 'a higher compound of gold be taken, it is reduced 



of the so-called fulminating gold, which contains gold, chlorine! hydrogen, nitrogen, 

 and oxygen, but its formula is not known with certainty. It is probably a sort of am- 

 Tttonio-metallic compound, Au a O3,4jJH 3 , or amide (like the- mercury compound). This 

 precipitate explodes at 140, but when left in the presence of solutions containing am- 

 monia it loses all its chlorine and becomes non-explosive. In this form the composition 

 Au.iO 3 ,2NH 5 ,H 2 O is ascribed to it, but this is- uncertain. Auric sulphide, Au 2 S 3 , is 

 obtained by the action of hydrogen sulphide on a solution of auric chloride, and also 

 directly by fusing sulphur with gold. It has an acid character, and therefore dissolves 

 in sodium arid ammonium sulphides. 



35 Many double salts of suboxide.of gold belong to the type AuX for instance, the 

 cyanide corresponding to the type AuKX 2 , like PtK 2 X 4 , with which we became acquainted 

 in the last chapter. We will enumerate several of the representatives of this class of 

 compounds. If auric chloride, AuCl 3 , be mixed with a solution of sodium thiosulphate, 

 the gold passes into a colourless solution, which deposits colourless crystals, con- 

 taining a double thiosulphate of gold and sodium, which are easily soluble in water 

 but are precipitated by alcohol. The composition of this salt is Na 3 Au(S 2 O 3 ) 2 ,2H 2 O.' 

 If the sodium thiosulphate be represented as NaS 2 O 5 Na, the double salt in question 

 will be AuNa(S 2 O 3 Na) 2 ,2H 2 O, according to the type AuNaX 2 . The solution of this 

 colourless and easily crystallisable salt has a sweet taste, and the gold is not separated 

 from it either by ferroms sulphate or oxalic acid. This salt, which is known as Fordos 

 and Gelis's salt, is used in medicine .and photography In general, aurous oxide 

 exhibits a distinct inclination to the formation of similar double salts, as we Baw also 

 with PtX 2 for example, it forms similar salts with sulphurous acid. Thus if a solution 

 of sodium sulphite be gradually added to a solution of oxide of gold in sodium 

 hydroxide, the precipitate at first formed re-dissolves to a colourless solution, which 

 contains the double salt Na 3 Au(SO 5 ) 2 =AuNa(SO 3 Na) 2 , The solution of this salt>, 

 when mixed with barium chloride, first forms a precipitate of barium sulphite, and 

 then a red barium double salt which corresponds with the above sodium salt. 



The oxygen compound of the type AuX, aurous oxide, Au 2 O, is obtained as a greenish 

 violet powder on mixing aurous chloride with potassium chloride in the cold. With 

 hydrochloric acid this oxide gives gold and auric chloride, and when heated it easily 

 splits up into oxygen and metallic gold. 



