26 THE ATOMIC WEIGHTS. 



Stas* in his experiments worked after two methods, which 

 gave, however, results concordant with each other and with 

 those of Marignac. 



In the first series of experiments Stas converted a known 

 weight of silver into nitrate, and then precipitated with 

 pure hydriodic acid. The iodide thus thrown down was 

 washed, dried, and weighed without transfer. By this 

 method 100 parts of silver were found to require of iodine : 



117.529 

 117-536 



Mean, 117.5325, =b .0024 



In the second series a complete synthesis of silver iodide 

 from known weights of iodine and metal was performed. 

 The iodine was dissolved in a solution of ammonium sul- 

 phite, and thus converted into ammonium iodide. The 

 silver was transformed into sulphate and the two solutions 

 mixed. When the precipitate of silver iodide was com- 

 pletely deposited the supernatant liquid was titrated for the 

 trifling excess of iodine which it always contained. As the 

 two elements were weighed out in the ratio of 127 to 108, 

 while the atomic weight of iodine is probably a little under 

 127, this excess is easily explained. From these experi- 

 ments two sets of values were deduced; one from the 

 weights of silver and iodine actually employed, the other 

 from the quantity of iodide of silver collected. From the 

 first set we have of iodine for 100 parts of silver : 



"7-5390 



117.5380 

 117.531S 

 117-5430 

 117.5420 

 117.5300 



Mean, 1 17-5373. ± -ooi5 



From the weight of silver iodide actually collected we 



* Aronstein's Translation, pp. 136, 152. 



