383 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



Aveight under consideration/ They analyzed pure mercuric solenide, 

 which had heen repeatedly sublimed and was well crystallized. Their 

 method of manipulation lias already been described in the cliapter upon 

 mercury. These percentages of Hg in HgSe w^ere found: 



71.726 

 71.731 

 71.741 



Mean, 71.7327, ± .003 



Hence Se= 78.88. 



The next determinations were made by Dumas/ w^ho returned to the 

 original method of Berzelius. Pure selenium was converted by dry 

 chlorine into SeCl4, and from the gain in weight the ratio between Se 

 and CI was easily deducible. I include Berzelius' single experiment, 

 w^hich I have already cited, and give in a third column the quantity of 

 chlorine absorbed by 100 parts of selenium : 



1.709 grm. Se absorb 3.049 grm. CI. 178.409 



1.810 " 3.219 " 177.845 



1.679 " 3.003 " 178.856 



1.498 " 2.688 " 179.439 



1.944 " 3.468 " 178.395 



1.887 " 3.382 " 179.226 



1.935 " 3.452 " 178.398 



179.000— Berzelius 



Mean, 178.696, ± .125 



Dumas' figures alone give Se= 79.39. 



The question may here be properly asked, whether it would be possible 

 thus to form SeCl4, and be certain of its absolute purity? A trace of 

 oxychloride, if simultaneously formed, Avould increase the apparent atomic 

 weight of selenium. In point of fact, this method gives a higher value 

 for Se than any of the other processes which have been adopted, and 

 that value has the largest probable error of any one in the entire series. 

 A glance at the table which summarizes the discussion at the end of 

 this chapter will render this point sufficiently clear. 



Still later, Ekman and Pettersson ^ investigated several methods for 

 the determination of this atomic weight, and finally decided upon the 

 two following: 



First, pure silver selenite, AgoSeOg, was ignited, leaving behind metallic 



' .lourn. prakt. Chem., 55, 202. 1852. 

 = Ann. Chem. Pharm., 113, .32. 186(). 

 •'' Ber. Deutsch. chem. Gesell., n, 1210, 1876. PuMishoil in detail by the society at Tpsnln. 



