170 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The results of the four analyses of the third series give the follow- 

 ing figures for the per cent of silver in the bromide : 57.443, 57.443, 

 57.447, and 57.443. The mean of these results is 57.444 ; Stas hav- 

 ing found 57.445. It will be remembered that the hydrobromic acid 

 from which the zincic bromide was made gave precisely similar results. 

 This identity proves that the analytical work was without fault, and 

 that argentic bromide does not possess the slightest tendency to occlude 

 zincic bromide when precipitated from dilute solutions. The analysis 

 of the hydrobromic acid proved that the material was free from chlorine 

 and iodine. 



Accordingly the rather large variations in the results must be due 

 wholly to the original condition of the samples of zincic bromide. It 

 remained therefore, to make a final series of determinations upon zincic 

 bromide from which water should have been absolutely excluded ; 

 and since one of us was unfortunately called away, this series was 

 made by the other alone. 



Final Series of Determinations. 

 By Theodore William Richards. 



One determination of the final series. No. 17, was made with the 

 old zincic bromide in the new apparatus. The others were all made 

 from new material prepared from electrolytic zinc and pure bromine, 

 instead of from zincic oxide and hydrobromic acid. The electrolytic 

 zinc was prepared with great care in the following manner : An excess 

 of " pure" zinc was treated with somewhat dilute pure sulphuric acid 

 at 80° until upon dilution a marked amount of basic salt was formed. 

 About three hundred grams of zinc had been dissolved ; and this di- 

 luted solution was allowed to stand for many hours in contact with 

 the zinc and the basic salt. After filtering, clean pieces of zinc were 

 added to the solution; and no further metallic precipitate formed upon 

 the zinc. The solution was then decanted and treated with a small 

 amount of sulphuric acid and much hydric sulphide. After some time 

 the pure white precipitate was separated by decantation and filtration, 

 and the solution was oxidized by an excess of pure chlorine. To this 

 solution was then added enough very pure sodic carbonate to form a 

 slio-ht precipitate, and the mixture was allowed to stand several days 

 with occasional stirring. The pure white precipitate, which must 

 have contained any trace of iron remaining, was filtered off, and the 

 sulphate of zinc was crystallized three times successively from hot 

 water. 



