136 RESEARCHES UPON ATOMIC WEIGHTS. 



pentoxide is heated to 240°, for traces of free iodine can be detected in air that 

 has been passed over iodine pentoxide at that temperature. That no appreciable 

 quantity of iodine could remain in the pentoxide was shown by the fact that the 

 solutions of the pentoxide were always absolutely colorless even when concen- 

 trated. Furthermore it was found by experiment that a mere trace of iodine 

 could be detected by its color in such a solution. About o.oi per cent of iodine 

 in the pentoxide would be necessary to raise the observed atomic weight of 

 silver by 0.0 1 unit. 



Iodine heptoxide might result from either the presence of periodic acid in the 

 iodic acid or from auto-oxidation of the pentoxide during the heating. Both 

 possibihties are wanting in plausibility, for it is not at all probable in the light 

 of the known instability of the heptoxide that the latter substance could have 

 withstood the high temperature of heating. 0.025 P^'^ cent of heptoxide in the 

 iodic acid would be necessary to lower the observed atomic weight of silver by 

 0.01 unit. 



It is intended to pursue farther the study of iodic acid in this laboratory, 

 by the preparation of this substance by other methods than the action of nitric 

 acid on iodine. Furthermore, since a very exact knowledge of the ratio of the 

 atomic weights of silver and iodine is necessary for the computation of the pro- 

 portion of iodine in the pentoxide, it is intended to investigate farther the com- 

 bining proportions of silver and iodine, especially by a method as nearly as 

 possible like that employed in this research. 



The results of this research may be briefly summed up as follows: 



1. The preparation of pure iodic acid is described. 



2. The existence of the compound I2O5.HIO3 is confirmed. 



3. It is shown that, while iodic acid may be almost completely converted 

 to pentoxide by heating at 240°, a small proportion of water remains, which is 

 constant for definite conditions of heating. 



4. It is shown that silver iodide occludes silver sulphate and that sulphur 

 dioxide may not be used as a reducing agent if the iodine is to be precipitated 

 by means of silver. 



5. Hydrazine salts are found to be suitable reducing agents. 



6. The specific gravity of iodine pentoxide at 25° referred to water at 4° is 

 found to be 4.80. 



7. It is shown that iodine pentoxide does not adsorb appreciable amounts 

 of air. 



8. The ratio of silver to iodine pentoxide is found to be 0.646230. 



9. Upon this basis, if oxygen is assumed to be 16.000, and if the ratio of silver 

 to iodine is assumed to be 0.849943, the atomic weight of silver is 107.850 and 

 that of iodine is 126.891. 



