470 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



Hence, 



From ratio 4 Os = 190.374. it .0299 



" 5 190.832. ± .0417 



" 3 191.229. ± .0173 



" 1 191.260, ± .0200 



" 2 192.216, ± .0852 



General mean. Os = 191.067, ± .0114 



A modern determination of the atomic weight of osmium seems to be 

 desirable. 



lEIDIUM. 



The only early determination of the atomic weight of iridium was 

 made by Berzelius/ who analyzed potassium iridichloride by the same 

 method employed with the platinum and the osmium salts. The result 

 found from a single analysis was not far from Ir = 196.7. This is now 

 known to be too high. I have not, therefore, thought it worth while to 

 recalculate Berzelius' figures, but give his estimation as it is stated in 

 Roscoe and Schorlemmer's " Treatise on Chemistry." 



In 1878 the matter was taken up by Seubert,' Avho had at his disposal 

 150 grammes of pure iridium. From this he prepared the iridichlorides 

 of ammonium and potassium (NH^) JrCl^ and KalrClg, which salts were 

 made the basis of his determinations. The potassium salt was dried by 

 gentle heating in a stream of dry chlorine. 



Upon ignition of the ammonium salt in hydrogen, metallic iridium 

 was left behind in white coherent laminae. The results obtained were as 

 follows : 



Mean, 43.732, ±: .0035 

 Hence Ir= 193.395. 



The potassium salt was also analyzed by decomposition in hydrogen 

 with special precautions. In the residue the iridium and the potassium 



' Poggcnd. Annalcn, 13, 435. 1S28. 



^Ber. Deutsch. chem. Gesell.. 11, 1767. 187S. 



