ATOMIC WEIGHTS 381 



SELENIUM. 



The atomic weight of this element was first determined by Berzeiius/ 

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

 chloride were produced. Hence Se = 79.24. Further on these figures 

 will be combined with similar results by Dumas. 



We may omit, as unimportant for present purposes, the analyses of 

 alkaline selenates made by Mitscherlich and Nitzscli," and pass on to 

 the experiments published by Sacc * in 18-17. This chemist resorted to 

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

 unsatisfactory. First, he sought to establish the exact composition of 

 SeOj, both by synthesis 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: 



.6800 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, ± .032 



Hence Se = 78.68. 



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

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

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

 BaSeOs with sulphuric acid, and estimating the resulting quantity of 

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

 100 of BaSeOg : 



.5573 grm. BaSeOj gave .4929 grm. BaSO,. 88.444 



.9942 " .8797 '" 88.383 



.2351 " .2080 " 88.473 



.9747 " .8621 " 88.448 



Mean, 88.437, ± .013 



Hence Se = 78.59. 



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 



1 Poggend. Annalen, S, 1. 1S26. 



- Poggend. Annalen, 9, 623. 1827. 



^ Ann. Chini. Phys. (3). 21, lis*. 

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