268 Professor Johnston on the Iodides of Gold. 



If more terchloride be now added, the teriodide falls in the 

 form of a dark green powder, and continues to fall on every 

 successive addition till the solution becomes nearly colourless. 

 The precipitate may be washed with water without much de- 

 composition, but cannot be dried, either by exposure to the air 

 or by artificial heat, without considerable loss of iodine. I 

 have not therefore attempted any direct analysis of it. It is 

 soluble in hydriodic acid and in solutions of the iodides, with 

 partial decomposition and formation of the .{iroto-iodide, and 

 is decomposed by the alkalies and alkaline earths. Exposed 

 to the air, it gradually becomes yellow, changing into proto- 

 iodide, and ultimately into metallic gold. 



Double Iodide {lodo-aurate) of Gold and Potassium. — When 

 concentrated solutions of terchloride of gold and iodide of po- 

 tassium are mixed with agitation in the proportion of rather 

 less than one equivalent of the former to four of the latter and 

 set aside, crystals of this salt speedily begin to deposit them- 

 selves in long acicular prisms; or if a concentrated solution of 

 iodide of potassium be digested on teriodide of gold, the latter 

 is largely dissolved, and the solution becomes of a dark brown- 

 ish red, almost black, and, except in thin layers, opake. Set 

 aside, this solution deposits crystals sometimes an inch in length, 

 which are slender, perfectly black, opake even by candle- 

 light, possessed of a high degree of lustre, and having much 

 resemblance to crystallized schorl. They have the form of four- 

 sided doubly oblique truncated prisms, cleaving readily parallel 

 to the terminal plane, and longitudinally striated on one pair 

 of the faces, indicating, probably, a cleavage parallel to the 

 other pair. 



These crystals are soluble in a weak solution of an alkaline 

 iodide, or of hydriodic acid, but are partially decomposed by 

 pure water. At 150° Fahr. or under, they emit a faint odour 

 of iodine, and gradually become purple ; in the air, at com- 

 mon temperatures also, they lose iodine but very slowly, and 

 are otherwise permanent. They contain no water of crystalli- 

 zation. Heated in a close tube they therefore give off no 

 moisture, but emit violet fumes, and leave a skeleton of a gold 

 yellow colour, exhibiting the original form of the crystal. 



Dried at a temperature of about 100° Fahr., these crystals 

 lost when heated, in two experiments, 50*61 and 5r43 per cent, 

 of iodine, and left, after washing out the iodide of potassium, 

 26'46 and 26*95 per cent, of metallic gold. They consist 

 therefore of Calc. Exp. Mean. 



lequiv.gold 24*86 26*76 26*705 



3 equiv. iodine 47*349 50*97 51*02 



1 equiv. iodide of potassium 20-682 22*27 22*275 



92-891 100 100 



