48 THE ATOMIC WEIGHTS. 



The ratios upon which we must depend for the atomic weight of 

 iodine are exactly parallel to those used for the determination of bromine. 



To begin with, the percentage of oxygen in potassium iodate has been 

 determined by Millon.* In three experiments he found : 



22.46 

 22.49 



22.47 



Mean, 22.473, .5 



Millon also estimated the oxygen in silver iodate, getting the follow- 

 ing percentages : 



17.05 

 17.03 

 17.06 



Mean, 17.047, .005 



The analysis of silver iodate has also been performed with extreme 

 care by Stas.f From 76 to 157 grammes were used in each experiment, 

 the weights being reduced to a vacuum standard. As the salt could not 

 be prepared in an absolutely anhydrous condition, the water expelled in 

 each analysis was accurately estimated and the necessary corrections ap- 

 plied. In two of the experiments the iodate was decomposed by heat, 

 and the oxygen given off was fixed upon a weighed quantity of copper 

 heated to redness. Thus the actual weights, both of the oxygen and the 

 residual iodide, were obtained. In a third experiment the iodate was 

 reduced to iodide by a solution of sulphurous acid, and the oxygen was 

 estimated only by difference. In the three percentages of oxygen given 

 below, the result of this analysis conies last. The figures for oxygen are 

 as follows : 



16.976 



16.972 



16.9761 



Mean, 16.9747, d= .0009 



This, combined with Millon's series above cited, gives us a general 

 mean of 16.9771, .0009. 



The ratio between silver and potassium iodide seems to have been de- 

 termined only by Marignac.J and without remarkable accuracy. In five 

 experiments 100 parts of silver were found equivalent to potassium iodide 

 as follows : 



*Ann. Chim. Phys. (3), 9, 400. 1843. 

 fAronstein's translation, pp. 170-200. 

 I Berzelius' I^ehrbuch, 5th ed., 3, 1196. 



