92 RESEARCHES UPON ATOMIC WEIGHTS. 



RATIO OF SILVER TO SILVER IODIDE. 



PURIFICATION OF MATERIALS. 

 SILVER. 



This substance was purified, by methods which have been described previ- 

 ously in detail,^ by twice precipitating silver chloride from strongly acid solu- 

 tion, with subsequent reduction by means of invert sugar and sodium hydroxide 

 in each case, especially purified reagents being employed in the second precipi- 

 tation and reduction. The product, after fusion before a blowpipe upon a cru- 

 cible lined with the purest lime, was converted into electrolytic crystals and 

 these were fused in a current of hydrogen upon a boat of pure lime. The re- 

 sulting buttons of metal, after being freed from adhering lime by treatment with 

 dilute acid, were cut into fragments of from 3 to 5 gm. by means of a clean 

 chisel and anvil, and the iron adhering to the surface was removed by warm- 

 ing the pieces repeatedly with fresh portions of dilute nitric acid, until iron 

 could no longer be detected in the acid. The fragments were washed thoroughly 

 with the purest water, then with ammonia, and again with water as before. 

 They were allowed to dry in the air, and finally the last traces of moisture were 

 eliminated by heating in a vacuum, since silver which is heated to a high tem- 

 perature in the air has been shown by Stas to absorb considerable oxygen.^ 

 Silver prepared in a similar fashion by investigators in this laboratory has always 

 been found to be pure, and since two different samples were used in the following 

 work with identical results, there can be no doubt of the purity of that employed 

 here. Furthermore, one synthesis of silver chloride, carried out with a portion 

 of this silver by Wells in the investigation of Richards and Wells upon the 

 atomic weight of chlorine, yielded a value which is identical with the average 

 of other similar determinations made with his own silver.' 



IODINE. 



The chief impurities in commercial iodine are halogens of lower atomic 

 weight and iodide of cyanogen. These were removed by dissolving iodine in a 

 strong solution of half its weight of potassic iodide, and distilling the greater 

 portion of the iodine from a retort into a flask cooled with cold water. The 

 iodine thus obtained was next converted into hydriodic acid by covering it with 

 considerable water and passing through the solution a stream of hydrogen sul- 

 phide. This gas was generated by the action of dilute sulphuric acid upon 

 ferrous sulphide, and was purified by bubbling through three gas-washing 

 bottles containing water and through two towers filled with beads moistened 

 with water. The reaction between the hydrogen sulphide and the iodine results 



^ See pages 6 and 22. 

 * CEuvres Completes, 3, 125. 



' Pub. Car. Inst., No. 28, p. 65 (1905); Jour. Amer. Chetn. Soc, 27, 524; Zeit. anorg. 

 Chan., 47, 130. 



