268 THE ATOMIC WEIGHTS. 



SELENIUM. 



The atomic weight of this element was first determined by Berzelius,* 

 who, saturating 100 parts of selenium with chlorine, found that 179 of 

 chloride were produced. Further on these figures will be combined with 

 similar results by Dumas. 



We may omit, as unimportant for present purposes, the anal3^ses of 

 alkaline selenates made by Mitscherlich and Nitzsch.f and pass on to 

 the experiments published by Sacc | in 1847. This chemist resorted to 

 a variety of methods, some of which gave good results, while others were 

 unsatisfactory. First, he sought to estal)lish the exact composition of 

 SeO.^, both by sjnithesis and by analysis. The former plan, according to 

 which he oxidized pure selenium by nitric acid, gave poor results ; better 

 figures were obtained upon reducing SeO.^ with ammonium bisulphite 

 and hydrochloric acid, and determining the percentage of selenium set 

 free: 



.6Soo grm. SeO.^ gave .4828 grm. Se. 71.000 per cent. 



3.5227 " 2.5047 " 71.102 " 



4.4870 " 3-1930 " 71. 161 



Mean, 71.088, i .032 



In a similar manner Sacc also reduced barium selenite, and weighed 

 the resulting mixture of barium sulphate and free selenium. This pro- 

 cess gave discordant results, and a better method was found in calcining 

 BaSeOg with sulphuric acid, and estimating the resulting quantity of 

 BaSO^. In the third column I give the amounts of BaSO^ equivalent to 

 100 of BaSeO^ : 



•5573 grm- l^aSeO^ gave .4929 g'"'"- BaSO^. 88.444 



.9942 " .8797 " 88.383 



.2351 " .2080 " 88.473 



.9747 " .8621 " 88.448 



Mean, 88.437, zh .013 



Still other experiments were made with the selenites of silver and lead ; 

 but the figures were subject to such errors that they need no further dis- 

 cussion here. 



A few years after Sacc's work was published, Erdmann and Marchand 

 made with their usual care a series of experiments upon the atomic 

 weight under consideration. § They analyzed pure mercuric selenide, 

 which had been repeatedly sublimed and was well crystallized. Their 



* Poggend. Annalen, 8, i. 1826. 

 tPoggend. Annalen, 9, 623. 1S27. 

 J Ann. d. Chim. et d. Phys. (3), 21, irg. 

 § Jour, fiir Prakt. Chem., 55, 202. 1852. 



