SELENIUM. 269 



method of manipulation has already been described in the chapter upon 

 mercury. These percentages of Hg in HgSe were found : 



71.726 



71-731 



71.741 



Mean, 71.7327, zh .003 



The next determinations were made by Dumas * who returned to the 

 original method of Berzelius. Pure selenium was converted by dry 

 chlorine into SeCl^. and from the gain in weight the ratio between Se 

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

 which 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 



i.<198 " 2.688 " 179.439 



1.944 " 3.468 " 178.39s 



1.887 " 3-382 " 179.226 



1-935 " 3-452 " 178-398 



179.000 — Berzelius. 



Mean, 178.696, ± .125 



The question may here be pro})erly asked, whether it would be possi- 

 ble thus to form SeCl^, and be certain of its absolute purity ? A trace of 

 oxychloride, if simultaneously formed, would increase the apparent 

 atomic weight of selenium. In point of fixct, this method gives a higher 

 value for Se than any of the other ])rocesses 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 f investigated several methods for 

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

 two following : 



First, pure silver selenite, Ag^SeO.^ was ignited, leaving behind metallic 

 silver, which, however, sometimes retained minute traces of selenium. 

 The data obtained were as follows : 



Ag.^SeO^. Ag. Per ce?it. Ag. 



5.2102 3-2787 62.93 



5-9721 3-7597 62.95 



7.2741 4.5803 62.97 



7.5390 4-7450 62.94 



6.9250 4.3612 62.98 



7.3455 4.6260 62.98 



6.9878 4.3992 62.95 



Mean, 62.957, ± .005 



♦Ann. Chem. Pharm., 113, 32. i860. 



■f Ber. d. Deutsch. Chem. Gesell., 9, 1210. 1S76. Published in detail by the society at Upsala. 



