SELENIUM. 269 



method of manipulation has already been described in the chapter upon 

 mercury. These percentages of Hg in HgSe were found : 



71.726 



7r-73i 



71.741 



Mean, 71.7327, .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 4 , and from the gain in weight the ratio between Se 

 and Cl 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 : 



.709 grm. Se absorb 3.049 grm. Cl. 178.409 



.810 " 3.219 " 177.845 



.679 " 3.003 " 178.856 



.498 " 2.688 " 179-439 



944 " 3.468 " 178.395 



.887 " 3.382 " 179.226 



935 " 3.452 " 178.398 



1 79.000 Berzelius. 



Mean, 178.696, .125 



The question may here be properly asked, whether it would be possi- 

 ble thus to form SeCl 4 , 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 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 f investigated several methods for 

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

 two following : 



First, pure silver selenite, Ag. 2 Se0 3 was ignited, leaving behind metallic 

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

 The data obtained were as follows : 



Ag^SeO^. Ag. Per cent. 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, d= .005 



*Ann. Chetn. Pharm., 113, 32. 1860. 



-f Ber. d. Deutsch. Chem. Gesell., 9, 1210. 1876. Published in detail by the society at Upsala. 



