A REVISION OF THE ATOMIC WEIGHT OF IODINE. 1 13 



chemists, determined the ratio Agl: AgCl, in eight closely agreeing analyses. 

 One synthesis was made by precipitating a weighed amount of silver with 

 hydriodic acid, and one by heating a weighed amount of silver in iodine. A 

 second paper ^ by the same authors contains a recalculation of their results with 

 a different value for chlorine, and a critical discussion of their own, Ladenburg's, 

 and my work. 



In this second paper two main criticisms of my investigation are made. One 

 of these concerns the validity of the conclusion that ordinary iodine does not 

 contain an undiscovered halogen element. This criticism is founded upon 

 a mistaken understanding, however. My experiments were directed to prove 

 the existence or non-existence of an element of higher atomic weight thali iodine, 

 which, if its properties were those to be expected from the properties of the 

 other members of the halogen family, would have been set free from solutions 

 of its compounds by iodine, and hence would have accumulated in the first 

 fraction of the fractional separation (see page 93). The existence of an un- 

 known halogen of lower atomic weight than iodine was not considered. 



Although Kothner assumes the possibiHty that such an element exists, it is 

 obviously improbable that any halogen of lower atomic weight could have re- 

 mained in the purified samples of iodine which were employed in my experi- 

 ments; since those specimens which received even the least purification were 

 thrice distilled from an iodide, the iodide having been made in each distilla- 

 tion from nearly half the iodine from the previous distillation. 



In the second place, it is maintained that volatilization of silver halides takes 

 place when silver iodide is heated in a current of chlorine. This is undoubtedly 

 true, unless precautions are taken to prevent the volatilization. Silver iodide is 

 much more volatile at its fusing temperature than silver chloride; and if the 

 original silver iodide is fused before the current of chlorine or bromide is begun, 

 a loss may take place by volatiHzation or possibly by spattering. In my own 

 work, however, fusion of the salt was always avoided until the greater part of 

 the change had taken place. Proof that no volatilization of silver halides actually 

 occurred in my experiments has already been given (page 105). It may be added 

 that the exact agreement of the results of Series V and VI is additional evi- 

 dence in the same direction; for any loss of silver bromide, which is the most 

 volatile of the three halides, would have tended to raise the results of Series V 

 above those of Series VI. 



Kothner's average result in the series Agl: AgCl, when recalculated upon the 

 basis of chlorine 35.457, becomes 126.915. The slight difference between this 

 value and the final result of the research described in this paper may be explained 

 in several possible ways. A perusal of Kothner's paper does not make clear 

 whether or not fusion of the silver iodide took place at the beginning of his ex- 

 periments, so that it is uncertain whether the gain in weight of the coil of glass 



^ Liebig's Ann., 337, 362 (1904). 



