340 SMITIISOXIAX MISCELLANEOUS COLLECTIONS VOL. 54 



CHEOMIUM. 



Concerning the atomic- weight of chromium there has been much dis- 

 cussion, and many experimenters have sought to establish the true value. 

 The earliest work upon it liaving any importance was that of Berzelius/ 

 in 1818 and 1826, whicli led to results much in excess of the correct 

 figure. His method consisted in precipitating a known weight of lead 

 nitrate with an alkaline chromate and weighing the lead chromate thus 

 produced. The error in his determination arose from the fact that lead 

 chromate, except when thrown down from very dilute solutions, carries 

 with it minute quantities of alkaline salts, and so has its apparent weight 

 notably increased. When dilute solutions are used, a trace of the pre- 

 cipitate remains dissolved, and the weight obtained is too low. In 

 neither case is the method trustworthy. 



In 1844 Berzelius' results were first seriously called in question. The 

 figure for chromium deduced from his experiments was somewhat over 

 56 ; but Peligot ^ now showed, by his analyses of chromous acetate and 

 of the chlorides of chromium, that the true number was near 53.5. 

 Unfortunately, Peligot's work, although good, was published with in- 

 sufficient details to be useful here. For chromous acetate he gives the 

 percentages of carbon and hydrogen, but not the actual weights of salt, 

 carbon dioxide, and water from which they were calculated. His figures 

 vary considerabl}', moreover — enough to show that their mean would 

 carry but little weight when combined with the more explicit data fur- 

 nished by other chemists. 



Jacquelain's ' work we may omit entirely. He gives an atomic weight 

 for chromium which is notoriously too low (50.1), and prints none of the 

 numerical details upon which his result rests. The researches which 

 particularly command our attention begin with those of Berlin.* His 

 starting point was nomial silver chromate; but in one experiment the 

 dichromate Ag^Cr^O^ was used. These salts, which are easily obtained 

 in a pure condition, were reduced in a large flask by means of hydro- 

 chloric acid and alcohol. The chloride of silver thus formed was washed 

 by decantation, dried, fused and weighed without transfer. The united 

 washings were supersaturated with ammonia, evaporated to dryness, and 

 the residue treated with hot water. The resulting chromic oxide was 



1 Schvveigg. Journ., 22, 53, and Poggend. AnnaL, 8, 22. 



^Conipt. Rend., 19, 609 and 734; 20, 1187; 21, 74. 



3 Compt. Rend., 24, 679. 1847. 



*Jouin. prakt. Chem., 37, 509, and 3S, 149. 1S46. 



