Professor Johnston on the Iodides of Gold. 267 



only O'l per cent., and was therefore nearly pure gold. An- 

 other large portion, containing some of these brighter scales 

 mixed with it, lost 24"86 per cent. 



It appears then that the crystalline scales which fall from a 

 hot solution of proto-iodide of gold are a mixture of this iodide 

 and of metallic gold to the amount of 12 or 13 per cent. 



A portion of the precipitate, prepared by adding terchloride 

 of gold to iodide of potassium in excess and afterwards wash- 

 ing with water, of a bright yellow colour with little lustre, lost 

 when heated 34*93 per cent. Obtained by this process, there- 

 fore, it contains 10 per cent, of metallic gold. 



Prepared by pouring a solution of iodide of potassium on 

 protochloride of gold, washing with water, drying in a cool 

 place on bibulous paper, it has a pale green colour. Heated 

 to redness, 10 346 grains, apparently of great purity, lost 38*85 

 per cent., almost the exact theoretical quantity. 



The proto-iodide of gold, therefore, is a pale green )X)wder, 

 which decomposes slowly in the atmosphere, is soluble in h}^- 

 driodic acid and in hot dilute solutions of the alkaline iodides, 

 precipitating from the latter on cooling in brilliant scales of 

 a gold yellow colour, intimately mixed with about 1 2 per cent, 

 of pure gold. At 150° Fahr. it is partially, and between 300° 

 and 400° wholly, decomposed; and it consists of 



Calc, Exp. 



Gold 24-86 61-167 61-15 



Iodine 15-783 38-833 38-85 



40-643 100 100 



II. Teriodide of Gold. Au + 3 I. 



When a solution of terchloride of gold is gradually added 

 to one of iodide of potassium the solution becomes dark- 

 coloured, and if agitated, the dark green precipitate which ap- 

 pears is nearly all redissolved. This continues till one equi- 

 valent of tex'chloride has been added for every four equivalents 

 of iodide of potassium, and there ai'e formed three equivalents 

 of chloride of potassium and one of the double iodide (iodo- 

 aurate) of gold and potassium, which, unless the solutions be 

 very concentrated, all remain in solution. The reaction is as 

 follows : 4 K I . Au CI3 = 3 K CI . K I + Au I3. 



This reaction, however, does not always take place exactly 

 as here represented. The teriodide is so easily decomposed 

 that, when precipitated, instead of being entirely redissolved 

 by the excess of the alkaline iodide, it is partly dect)mposed, 

 and a portion of the proto-iodide falls, the liberated iodine 

 being held in solution. 



2 H2 



