BAXTER. A REVISION OF THE ATOMIC WEIGHT OF IODINE. 435 



under way upon the atomic weight of iodine, with the purpose of deter- 

 mining the ratio between iodine and silver iodide, and the ratio between 

 all the halogens as described upon page 433 of this paper. 



In discussing the bearing of this investigation upon the results of earlier 

 work by other chemists, the experiments of Millon upon silver and potas- 

 sium iodates * and of Berzelius t and Dumas,t who converted silver iodide 

 into silver chloride, may be disregarded, since at the time the analyses 

 were made, quantitative analysis was in its infancy. Marignac's § value 

 for the atomic weight of iodine, 126.85, obtained from titration of weighed 

 amounts of silver with potassic iodide, and from syntheses of silver iodide 

 from weighed quantities of silver, may be accounted for by the supposition 

 that the iodine used in the experiments was not pure. To explain Stas's || 

 low value, 126.85, is a difficult matter. His iodine was purified by two 

 different methods, i. e. by once precipitating or distilling the iodine from 

 a strong solution of potassic iodide, and by precipitation of nitrogen iodide. 

 A third sample was purified by both methods. Since the material puri- 

 fied by each of the two methods gave identical results with that purified 

 by both methods, it is inconceivable that either method of purification 

 should not have been effective. Impurity in the silver or loss of silver 

 iodide are improbable causes of the discrepancy, for the weight of the 

 silver iodide produced was equal to the sum of the weights of the silver 

 and iodine employed. Richards and Wells have recently shown, however, 

 that Stas was not infallible, and in fact was capable of making serious 

 mistakes, such that his value for the atomic weight of sodium was two 

 tenths of a per cent too high, and that for the atomic weight of chlorine 

 three hundredths of a per cent too low, so that it is not at all surprising to 

 find that here also his work was faulty in some undiscovered particular. 



Ladeuburg's result, 1[ when calculated from the true value of the atomic 

 weight of chlorine, becomes 126.978, which agrees very closely with the 

 value deduced in this paper. His determinations were affected by several 

 small errors, so that the close agreement is somewhat the result of chance. 

 In the first place, porcelain crucibles, as has been pointed out before, gain 

 in weight when used for the conversion of silver iodide into silver chloride, 

 so that the weight of the silver chloride is somewhat uncertain. Fur- 



* Ann. (le Cliern. et de Phys. (3) 9, 400 (1843). 



t Ibid., (2) 40,430 (1829). 



t Ann. Chem. Pliarni., 113, 28 (1860). 



§ Berzelius' Lehrbuch, 5th Ed. 3, 1106. 



II fEuvres Completes, 1, 648. 



1 Ber. d. d. Chem. Gesell., 35, 2276. 



