20 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



which we made, judging from the analysis. The method of purifica- 

 tion just described is the most convenient and safest of all. The only 

 improvement which might be suggested is the use of a silver dish for 

 conducting the final precipitation. This improvement was tried in the 

 next preparation. 



Part of the silver, not electrolyzed, proceeding from the earlier 

 reduction with ammonic formiate and ammonic acetate (p. 18) was 

 dissolved in redistilled nitric acid. The silver nitrate, once more 

 crystallized, was dissolved in pure water, in a large silver dish. 

 Ammonic formiate was freshly prepared by passing pure ammonia gas 

 into redistilled formic acid. With it the silver nitrate was reduced in 

 a silver dish, but otherwise as described above, washed free from 

 ammonia and dried in the same silver dish. This sample was called W. 



There is no reason to believe that any of these samples of silver 

 contained any appreciable impurity, except the traces of included 

 mother liquor already alluded to. It is probable that Stas's silver 

 also was equally pure at this stage in the proceedings. The subse- 

 quent treatment of the silver, in the effort to eliminate the mother 

 liquor and prepare the metal for weighing, seemed to be in our experi- 

 ments the only cause of difference in the quantitative behavior of our 

 samples, and there is every reason to believe that Stas introduced 

 during this treatment the impurities which have rendered a revision 

 of his work imperative. 



Because of this importance of the subsequent treatment, it is 

 necessary to record a very detailed statement of the precise operations 

 of the present research in this particular, and to point out the differ- 

 ences between these operations and the methods of Stas. 



Everyone must agree that fusion alone is the only safe method of 

 eliminating wholly the liquid included in the cells of the crystals, 

 whether these crystals be produced by electrolytic or chemical reduc- 

 tion. The difficulty is to find an environment for the silver during its 

 fusion from which it will not absorb or retain foreign substances. 



The question presents a two-fold difficulty, because both the solid 

 support of the silver and the atmosphere which surrounds it must be con- 

 sidered. The former of these difficulties may be discussed first. 



In what vessel may the silver be fused, in order to avoid con- 

 tamination ? Stas, in his posthumous work, recommends fusion in 

 a cupel of basic calcic phosphate, in the flame of a blast lamp, and 

 subsequent ignition in hydrogen. This process, while possibly as 

 good as he contends, may nevertheless be a dangerous one. Everyone 

 knows the risk to a platinum crucible of igniting in it a phosphate 

 with reducing material, and yet Stas took no steps to prove the absence 



