A REVISION OF THE ATOMIC WEIGHT OF IODINE. 105 



RATIO OF SILVER IODIDE TO SILVER BROMIDE. 



In the first place eight experiments were made in which the iodide was heated 

 in a current of air and bromine. The air was purified by being conducted suc- 

 cessively over beads moistened with silver nitrate solution, sodium carbonate, 

 and finally concentrated sulphuric acid which had been heated to its boiHng 

 point with a small quantity of recrystallized potassium dichromate to eHminate 

 volatile and oxidizable impurities. Four different samples of bromine were em- 

 ployed, each one of which had been thrice distilled from a solution of a bromide, 

 in each distillation the bromide having been made from a portion of the product 

 of the previous distillation. Chlorine must have been completely separated in 

 this way. The presence of iodine in the bromine was of less importance. Nev- 

 ertheless each sample, while in the form of hydrobromic acid, was freed from 

 iodine by boiling with several small portions of potassic permanganate. 



The apparatus for purifying the air and saturating the air with bromine was 

 so constructed that the gases came in contact only with glass. The quartz cru- 

 cibles were always contained in large porcelain crucibles during the heating in 

 bromine, as well as in the initial fusion of the silver iodide with iodine. Vola- 

 tilization of the silver halides was prevented by heating very gently at first, at a 

 temperature insufiicient to fuse the mixture; then, when the greater part of the 

 iodine had been replaced, till the silver bromide barely fused. Moreover, the cru- 

 cible was very deep and the current of gases very slow, so that any volatilized 

 silver halides had opportunity to condense upon the cool walls of the crucible. 



That no loss from volatilization actually took place was certain for two reasons. 

 In the first place, the weight of the bromide in most cases became constant within 

 a few hundredths of a miUigram after once being heated, although subsequently 

 the salt was maintained at a temperature slightly above its melting point for at 

 least an hour. In the second place, the perforated cover and hard-glass delivery- 

 tube for the bromine, when rinsed with ammonia and the solution treated with 

 hydrochloric acid in excess, gave no visible trace of opalescence. Before the salt 

 was allowed to solidify it was maintained above its fusing point in air for a few 

 minutes to eliminate dissolved bromine. 



The silver bromide resulting from each analysis was converted into silver chlo- 

 ride by heating in a current of pure dry chlorine as described on page 102. Here 

 also constant weight within a very few hundredths of a milligram was easily 

 obtained, and as before the perforated cover and inlet tube were free from weigh- 

 able amounts of chloride. It has already been shown that silver chloride and 

 silver iodide, when fused in an atmosphere of the corresponding halogens and 

 then in air, retain none of the halogen. The quartz crucibles remained almost 

 absolutely constant in weight during these experiments. From the ratio be- 

 tween the original silver iodide and the silver chloride the atomic weight of iodine 

 was calculated. All completed analyses are recorded in the tables. Several 

 were lost, owing to breaking of the crucibles. The vacuum correction for silver 

 bromide is assumed to be -1-0.000041. 



