A REVISION OF THE ATOMIC WEIGHTS OF IODINE AND SILVER. 121 



cleansed by scrubbing with sand and etching with nitric acid,'they were dissolved 

 in nitric acid and the silver was precipitated as metal by means of ammonium 

 formate made from redistilled ammonia and formic acid. This precipitate in 

 turn was thoroughly washed and the metal was fused in the flame of a carefully 

 cleaned blast lamp in a crucible of lime made from purified calcium nitrate and 

 carbonate. After being cleansed as before, the buttons were electrolyzed and 

 the electrolytic crystals, after being washed and dried, were fused in a current 

 of pure electrolytic hydrogen in an unglazed porcelain boat lined with the purest 

 lime.* The resulting buttons were cut into fragments of from 3 to 6 gm. with 

 a fine jeweller's saw, etched with small portions of dilute nitric acid until the 

 acid used in etching was free from iron, and finally the fragments were washed 

 and thoroughly dried in a vacuum at about 400°. Two different specimens of 

 silver were prepared in essentially the same way, Samples A and B. 



CONVERSION OF IODIC ACID INTO IODINE PENTOXIDE. 



The conversion of the iodic acid into iodine pentoxide was effected by heating 

 the substance in a current of dry gas. At the beginning of the research the hope 

 was entertained that the final product might be sublimed. In no experiment, 

 however, have we been able to detect the least trace of sublimation, even when 

 the pentoxide was heated to the decomposition point. Iodic acid may be made 

 to lose all its water of composition without fusion, and consequently it was to be 

 expected that under these conditions dehydration could be made nearly if not 

 quite complete. T. W. Richards has already pointed out that a solid which 

 loses water of composition without fusion is left in the form of a skeleton, from 

 the innermost interstices of which the water vapor may escape, while, if fusion 

 takes place during drying, a portion of the original salt may be enclosed within 

 an impervious coating of anhydrous salt so that escape of water is impossible.^ 



It is commonly stated that the dehydration of iodic acid takes place in two 

 stages, two- thirds the water being lost below 130° and the remainder at about 

 200°.' The acid itself, if heated rapidly, melts at no" with the separation of 

 the solid phase I2O5.HIO3, although this second phase shows no indication of 

 melting up to the temperature of the second stage in the dehydration. Our 

 earlier experience was not in accordance with the above statements. The acid, 

 when heated to about 110°, lost all its water of composition, at this tempera- 

 ture. This was shown conclusively in one experiment by heating a weighed 

 amount of iodic acid for some time at about 1 10° and then reweighing. The 



* Fusion in hydrogen (Baxter: Proc. Amer. Acad., 39, 249 [1903]) is probably safer than 

 fusion in a vacuum, since it eliminates the possibility of the taking up of sulphur by the silver 

 from the rubber ring used in fitting the hollow brass stoppers into the porcelain fusion tube. 

 Richards and Wells have shown that silver fused in hydrogen is certainly as pure as any other. 



2 Zeit. physik. Chem., 46, 194 (1903). 



' For a discussion of this subject see GroschufF, Zeit. anorg. Chem., 47, 333 (i9"5)- 



