PURPLE OF CASSIUS. 677 



it explodes most easily with a loud report, accompanied by a 

 feeble flame. It may be exploded by a heat a little above the 

 boiling point of water, or by the blow of a hammer. Its com- 

 position has not been certainly determined, but if the ammonia 

 is present in double the proportion that would contain the hy- 

 drogen necessary to burn the oxygen of the auric oxide, which 

 Berzelius considers probable, its constituents may be Au 2 O 3 4- 

 2NH 3 + HO. The affinity of auric oxide for ammonia is so great, 

 that it takes that alkali from all acids. Thus, when auric oxide 

 is digested in sulphate of ammonia, fulminating gold is formed, 

 and the liquid becomes acid. 



Purple of Cassius. When protochloride of tin is added to a 

 dilute solution of gold, a purple-coloured powder falls, which 

 has received that name. It is obtained of a finer colour, when 

 protochloride of tin is added to a solution of the perchloride of 

 iron, till the colour of the liquid has a shade of green, and add- 

 ing this liquid, drop by drop, to a solution of perchloride of gold 

 which is free from nitric acid, and very dilute. After 24 hours, 

 a brown powder is deposited, which is in a small degree trans- 

 parent and purple-red by transmitted light. When dried and 

 rubbed to powder, it is of a dull blue colour. Heated to red- 

 ness it loses a little water, but no oxygen, and retains its for- 

 mer appearance. If washed with ammonia on the filter while still 

 humid, it is dissolved, and a purple liquid passes through, 

 which rivals the hypermanganate of potash in beauty. From 

 this liquid, the colouring matter very gradually separates, weeks 

 elapsing before the upper strata of the liquid become colour- 

 less, but it is precipitated more rapidly when heated in a close 

 vessel between 140 and 180'. The powder of Cassius is inso- 

 luble in solutions of potash and soda. It may also be formed, 

 by fusing together 2 parts of gold, 3i parts of tin and 15 parts 

 of silver, under borax, to prevent the oxidation of the tin, and 

 treating the alloy with nitric acid to dissolve out the silver; a 

 purple residue is left containing the tin and gold that were 

 employed. 



The powder of Cassius is certainly, after ignition, a mixture 

 of peroxide of tin and metallic gold, from which the last can 

 be dissolved out by aqua regia, while the peroxide of tin is left ; 

 and the last mode of preparing it, favours the idea that its con- 

 stitution is the same before ignition. But its property to dis- 

 solve in ammonia, and the fact that mercury does not dissolve 



