A REVISION OF THE ATOMIC WEIGHTS OF IODINE AND SILVER. 1 27 



The platinum boat, containing about 25 gm. of acid which had been inocu- 

 lated with the second phase, was freed from water by heating at 100°, and then 

 at 220° + in the usual way. It was then bottled and weighed. Next the boat 

 was transferred to a long hard-glass tube very carefully ground into the socket 

 of the bottling apparatus and was heated to 240° for 4 hours in a current of dry 

 air. Toward the end of the heating all of the apparatus beyond the phosphorus 

 pentoxide drying tube was gently heated with a Bunsen flame in order to dis- 

 lodge any adsorbed water from the inside surface of the glass. Although by 

 far the greater part of the iodine formed in the decomposition condensed in the 

 hard-glass tube, a small quantity of iodine vapor was always carried along by 

 the current of gases.^ For the absorption of this iodine vapor a small hard-glass 

 tube containing small electrolytic crystals of silver which had been dried by 

 heating to about 400° in a vacuum was carefully ground on the end of the first 

 tube. This silver tube was attached during the decomposition of the pentoxide 

 and was heated to very dull redness. The column of metallic silver was several 

 inches in length, and although considerable silver iodide was produced at the 

 end of the column nearest the decomposition tube, the silver at the other 

 end of the column remained perfectly bright through all the determinations, 

 showing the absorption of iodine to have been complete. 



The U -tubes for the absorption of water were provided with glass stopcocks 

 and were filled with phosphorus pentoxide which had been freshly sublimed in 

 a current of oxygen. They were weighed by substitution, with the use of a 

 similar tube as counterpoise. Before being weighed, the tubes were wiped with 

 a damp cloth and were allowed to stand near the balance case for at least an 

 hour. The tubes were weighed with one stopcock open. The balance was pro- 

 vided with a few milligrams of radium bromide of radio-activity loooo to dispel 

 electrical charges. Under these conditions no difficulty was experienced in 

 weighing the tubes within a few hundredths of a milligram, since they quickly 

 came to constancy in the balance case and retained their weights unchanged 

 for days at a time. Two phosphorus pentoxide tubes were used in the first ex- 

 periments, but since the gain in weight of the second tube was found to be neg- 

 ligible in all cases where it was used, the second tube was omitted in the later 

 experiments. Still another phosphorus pentoxide tube was placed beyond the 

 weighed tubes as a protection against the back diffusion of moisture. Blank 

 experiments were usually run after the collection of the water resulting from the 

 decomposition. Frequently no gain in weight of the tube in these blank ex- 

 periments could be detected and in no case did the gain iti weight amount to 

 more than o.i mg. 



The decomposition of the iodine pentoxide was effected by removing the alumi- 

 num oven and heating the tube to the temperature of decomposition of the 



* Baxter, Hickey, and Holmes, " The Vapor Pressure of Iodine," Jour. Amer. Chem. Soc, 

 39, 127 (1907). 



