A REVISION OF THE ATOMIC WEIGHT OF IODINE. 1 03 



the apparatus for generating chlorine was constructed wholly of glass with the 

 exception of the joint between this hard-glass tube and the drying tube. The 

 chlorine did not come in contact with the rubber tube used in making the con- 

 nection tight, however, for the hard-glass tube telescoped into the drying tube 

 for some distance and the joint was sealed with concentrated sulphuric acid. 

 In order to avoid volatilization of the silver salts, the heat applied was very 

 gentle and only sufficient to fuse the mixed haHdes. Even at this temperature 

 the iodine was rapidly replaced. Heating in chlorine was continued some time 

 after the color of the iodine vapor had ceased to be visible. Then the Rose 

 cover was replaced by the ordinary cover and the silver chloride was kept fused 

 for several minutes, with occasional lifting of the cover, so as to drive off any 

 chlorine which might have been dissolved by the fused salt. Although fused 

 silver chloride, when cooled in chlorine, dissolves this gas very appreciably, no 

 evidence was found that any remained in the solidified salt when it was heated 

 for a short time in air. In one experiment 18 gm. of the chloride after the usual 

 treatment were re-fused in air with a loss in weight of only 0.00002 gm. The 

 same chloride, when fused and cooled in chlorine, gained 5 mg. 



It was soon discovered that a porcelain crucible, when used for the conversion 

 of silver iodide into silver chloride, gradually gained in weight. In the first ex- 

 periment this gain was over 3 mg., and in several subsequent experiments with 

 the same crucible, amounted to slightly less than a milligram in each case. This 

 gain did not take place to the slightest extent when the crucible was heated alone 

 in chlorine, nor after the reaction was complete, for the weight of the crucible 

 and silver chloride very soon became constant. Probably it was due partially 

 to solution of the silver salt in the glaze, as was shown by slight discoloration on 

 the bottom of the crucible. Possibly, however, it was caused by the attacking 

 of the glaze by the "nascent" chlorine and iodine set free in the process, per- 

 haps with evolution of oxygen. At any rate too great an uncertainty existed 

 as to the weight of the crucible at the end of the experiment. Accordingly quartz 

 crucibles were next employed for the same purpose. These crucibles behaved 

 in an ideal fashion, for they remained practically constant in weight through 

 the process. It was only necessary to rotate the crucibles during the solidifica- 

 tion of the fused salts in order that the salt might solidify in a thin layer on 

 the sides of the crucible. Neglect to do this almost invariably resulted in the 

 cracking of the crucible. 



After the first weighing of the crucible, it was again heated in chlorine for an 

 hour, and again cooled and weighed. In no case did a loss in weight of more 

 than 0.1 mg. take place, showing both that the iodine was completely replaced, 

 and that no silver chloride had volatilized. The following five experiments were 

 completed with quartz crucibles in the manner described. A vacuum correc- 

 tion of -f- 0.000071 is applied to every apparent gram of silver chloride.^ 



1 From the density 5.56 determined by Richards and Stull, not yet published, the weights 

 being assumed to have the specific gravity 8.3. 



