266 Professor Johnston on the Iodides of Gold. 



rictional, has been the great drawback on their being more 

 extensively used by the faculty. 



Seventh. The great portability of my machine makes it very 

 convenient for travelling, as there is a mahogany case which 

 slides upon 7?i against i, where it locks atn\ there is a closet in 

 the case, into which all the apparatus, figg. 2, 3, 4, 5, pack. 



Edward M. Clarke, Magnetician. 



No. 9, Agar Street, West Strand, London. 



LV. On the Iodides of Gold, By James F. W, Johnston, 

 Esq., A.M., F.R.S.E., 4'c. Sfc, Reader in Cliemislry and Mi- 

 neralogy in the University of Durham.* 



I. Proto-iodide of Gold. Au + I. 

 "l^HEN iodide of potassium in solution is added in ex- 

 ~ ' cess to a solution containing terchloride of gold, a pre- 

 cipitate is obtained of a greenish yellow colour, while the su- 

 pernatant liquid becomes dark-coloured from the excess of 

 iodine held in solution. If the terchloride be in excess, the 

 precipitate has a blueish gray colour, being a mixture of the 

 vellow powder with iodine, or with the teriodide of gold. If 

 the precipitate be heated in the dark-coloured mother liquor, 

 it diminishes in quantity, and on cooling gradually separates 

 in minute golden yellow crystalline scales of great beauty, ex- 

 hibiting apparently triangular and square faces, and not infe- 

 rior in lustre to iodide of lead crystallized from a similar hot 

 solution. 



This beautifulyellow compound consists chiefly of proloiodide 

 of gold. According to the analysis of Pelletier [Ann. de Chim. 

 et de Phys., vol. xv. p. 116) it contains 34 per cent, of iodine. 

 Exposed to the air and light, it is gradually decomposed, and 

 more readily when moist, giving off" iodine. At 150° Fahr. 

 it begins to give off iodine; at 230° Fahr., it undergoes almost 

 total decomposition, losing only one per cent, additional when 

 heated to 400° Fahr. Two portions prepared at different 

 times, lost when decomposed by heat, 33"96 and 34*43 of iodine 

 respectively. After exposure to the air for ten days it lost 

 only 9*6 per cent. 



A compound of one atom of gold with one of iodine should 

 contain 38'83 per cent, of iodine. The low temperature at 

 which the compound is decomposed rendering it probable that 

 the crystallized portions analysed by Pelletier and myself 

 might contain metallic gold, I collected first a heavier brighter 

 portion which collects at the bottom when the precipitate is 

 heated in the mother liquor : it lost when dried and heated 

 * Communicated by the Author. 



