ATOMIC WEIGHTS 93 



These figures were received too late to be used in the final reductions 

 of the fundamental ratios. 



THE SILVER-IODINE EATIO. 



The composition of silver iodide, first thoroughly investigated by 

 Marignac and Stas, has recently been the subject of elaborate researches. 



Marignac ^ dissolved weighed quantities of silver in nitric acid, and 

 precipitated the silver iodide with a solution of potassium iodide. He 

 gives the following weights, and the ratio of Agl to 100 parts of Ag: 



Mean, 217.511, ± .0036 



Corrected for weighing in air this becomes 217.5335. 



Three series of determinations are given by Stas," all with weights 

 corrected to a vacuum standard. 



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. 

 His figures are as follows : 



Mean, 217.5325, ± .0024 



In the second series a complete synthesis of silver iodide from known 

 weights of iodine and metal was effected. The iodine was dissolved in 

 a solution of ammonium sulphite, and thus converted into ammonium 

 iodide. The silver was transformed into sulphate and the two solutions 

 were mixed. When the precipitate of silver iodide was completely de- 

 posited 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 experiments 

 two sets of values were deduced; one from the weights of silver and 



1 Oeuvres Completes, 1, 86. 

 ^Oeuvres Completes, 1, 548-583. 



