448 PRINCIPLES OF CHEMISTRY 



aspect, gold presents, as is already seen from its general characteristics 

 given above, an example of the so-called noble metals i.e. it is 

 incapable of being oxidised at any temperature, and its oxide is 

 decomposed when calcined. Only chlorine and bromine combine 

 directly with it at the ordinary temperature, but many other metals 

 and non-metals combine with it at a red heat for example, sulphur, 

 phosphorus, and arsenic. Mercury dissolves it with great ease. It 

 dissolves in potassium cyanide in the presence of air ; a .mixture of 

 sulphuric acid with nitric acid dissolves it with the aid of heat, 

 although in small quantity. It is also soluble in aqua regia and in 

 selenic acid. Sulphuric, hydrochloric, nitric, and hydrofluoric acids 

 and the caustic alkalis do not act oh gold, but a mixture of hydro- 

 chloric acid with such oxidising agents as evolve chlorine naturally 

 dissolves it like aqua regia. 32 bi *. 



As regards the compounds of gold, they belong, as was said 

 above, to the types AuX 3 and AuX. Auric chloride or gold tri- 

 chloride, AuCl 3 , which is formed when gold is dissolved in aqua regia, 

 belongs to the former and higher of these types. The-solution of this 

 substance in water has a yellow colour, and jt may be obtained pure by 

 evaporating the solution in aqua regia to dryness, but not to the point 

 of decomposition. If the evaporation proceed to the point of crystal- 

 lisation, a compound of gold chloride and hydrochloric acid, AuHCl 4 , is 

 obtained, like the allied compounds of platinum ; but it easily parts 

 with the acid and leaves auric chloride, which fuses into a red-brown 

 liquid, and then solidifies to a crystalline mass. If dry chlorine be 

 passed over gold in powder it forms a mixture of aurous and auric 

 chlorides, but the aurous chloride is also decomposed by water into 

 gold and auric chloride. Auric chloride crystallises from its solutions 

 as AuCl 3 ,2H 2 O, which easily, loses water, and the dry chloride loses 

 two-thirds of its chlorine at 185, forming aurous chloride, whilst 



Al by 41 0< 7. The latter alloy is white. The alloy AuAl 2 has a characteristic purple 

 colour, and its melting point is 32'5 above that of gold, which shows it to be a definite 

 compound of the two metals. The melting points of alloys richer in Al gradually fall 

 to 660 that is, below that of aluminium (665). 



Heycock and Neville (1892), in studying the triple alloys of Au, Cd, and Sn, observed 

 a tendency in the gold to give compounds with Cd, and by sealing a mixture of Au and Cd 

 in a tube, from which the air had been exhausted, and heating it, they obtained a grey 

 crystalline brittle definite alloy AuCd. 



53 bis Calderon (1892), at the request of some jewellers, investigated the cause of a 

 peculiar alteration sometimes found on the surface of dead-gold articles, there appearing 

 brownish and blackish spots, which widen and alter their form in course of time. He 

 came to the conclusion that these spots are due to the appearance and development of 

 peculiar micro-organisms. (Aspergillus niger and Micrococcus cimbareus) on the gold, 

 spores of which were found in abundance on the cotton-wool in which the gold articles 

 had been kept. 



