BAXTER. — ATOMIC WEIGHT OF IODINE. 77 



by thrice converting it into hydriodic acid with hydrogen sulphide, 

 and then heating the hydriodic acid with a slight excess of potassium 

 permanganate which had been freed from chlorine by crystallization. 

 The final product of iodine was distilled once with steam, freed from 

 water by suction upon a porcelain Gooch crucible, and dried as far 

 as possible in a desiccator over concentrated sulphuric acid. It was then 

 sublimed once from porcelain boats in a current of pure dry air in a hard 

 glass tube, and then a second time from the hard glass tube into the 

 weighing tube, which was constructed as described on page 428 of 

 the first paper. This weighing tube remained constant in weight 

 within a few hundredths of a milligram in each experiment, and lost 

 in weight in all only one tenth of a milligram in the eight final analyses. 

 Next, the iodine was dissolved in sulphurous acid, and was then precipi- 

 tated by adding a solution of a slight excess of pure silver to the solu- 

 tion of hydriodic acid in a precipitating Mask. The silver iodide clotted 

 together very rapidly in the presence of the nitric acid and excess of 

 silver, and offered none of the difficulties met when the precipitation 

 is carried out with an excess of iodide, for it may be washed indefinitely 

 with pure cold water without showing the least tendency to pass into 

 colloidal solution. The silver iodide was collected upon a Gooch cruci- 

 ble, dried, and weighed. The loss on fusion was determined, and the 

 asbestos shreds in the filtrate were collected. Twelve determinations 

 carried out in this way with solutions varying from tenth to thirtieth 

 normal gave results from 126.93 to 126.98. The variations were 

 finally traced to the carrying down of silver nitrate by the silver iodide ; 

 for the more concentrated the solutions during the precipitation and the 

 greater the excess of silver employed, the lower was the observed atomic 

 weight of iodine. The occlusion of silver nitrate by silver halides is well 

 known in the cases of silver bromide and silver chloride, but in these 

 cases it is possible to wash out the occluded salts by oft-repeated treat- 

 ment with water. The difficulty in removing the silver nitrate in the 

 case of silver iodide is doubtless due to the lesser solubility of this halide. 

 Similar observations have been made by Kothner and Aeuer,* who found 

 that with fifth normal solutions the carrying down of silver nitrate is 

 very considerable, and that an excess of iodide converts this occluded 

 silver nitrate into silver iodide only very slowly if at all. They found, 

 however, that, if the precipitated silver iodide is- washed with ammo- 

 nia, the greater part of the occluded matter may be eliminated, owing 



*Liebig's Ann., 337, 123. 



