19.4 M. Stas on the Determination of Atomic Weights. 



still hot, and weighed* in a glass tube which was closed by a 

 stopper. The equivalent weight of silver was then changed by 

 method 1 into metallic silver, and the cold solution of silver 

 mixed with so much sulphuric acid that sulphurous acid pro- 

 duced no precipitate. The iodine was changed into hydriodic 

 acid by the addition of solution of sulphurous acid, and the latter 

 then added to the solution of silver, (b) Solution of ammonia 

 was saturated with sulphurous acid at 0°, and the solution mixed 

 with three times its volume of ammonia. The iodide was changed 

 into hydriodic acid; the rest of the process was the same as in 

 (a) . The deposition of iodide of silver is promoted by lengthened 

 agitation and then by warming. Iodine and silver were always 

 weighed out in the ratios of the old atomic weights (127 and 

 108). It was found that there was each time an excess of un- 

 combined iodine, which was estimated by standard solution of 

 silver. The whole of the iodide of silver was now collected, as 

 described above, and weighed. Six experiments, made with the 

 greatest care and with varying quantities of material (28-160 

 grms. of silver), showed that 100 parts of silver gave 217*5371 

 parts of iodide of silver, starting from the weight- of the iodine 

 and the silver used, — or 217*5335 parts, taking into account the 

 weight of the iodide of silver used. The numbers varied between 

 217*529 and 217-543. From the experiments in I., 



100 parts of silver yield 217*5325 iodide of silver, 

 217-5371 

 217-5335 ,. 



Marignac found 217-5335 . „ 



Bromide of silver was prepared by precipitating sulphate of 

 silver with hydrobromic acid. To prepare the latter, bromide of 

 potassium free from iodine is precipitated to the extent of three?- 

 fifths by means of nitrate of silver, and agitated for some time 

 with the liquid. In order to remove chlorine, the bromide of 

 silver is again shaken and finally heated with a solution of bro* 

 mide of potassium. Thereupon it is diffused in water and 

 treated with sulphuretted hydrogen. The solution of hydro- 

 bromic acid is freed from excess of sulphuretted hydrogen by 

 means of pure bromide of silver, and is then carefully distilled. 

 The concentrated hydrobromic acid thus obtained is colourless 

 and permanent in the air. It is only the acid which contains 

 iodine that thus becomes coloured. For precipitating the silver 

 a standard quantity of hydrobromic acid was used ; for an excess 

 of hydrobromic acid might dissolve some bromide of silver. 

 Bromide of silver precipitated in the cold is white. It was 

 washed with cold water, and then a current of aqueous vapour 

 was passed into the liquid to promote the deposition of bro- 

 * All weighings arc of course reduced to weights in vacuo* 



