238 THE ATOMIC WEIGHTS. 



CHROMIUM. 



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 having any importance was that of 

 Berzelius,* in 1818 and 1826, which 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 chro- 

 mate thus produced. The error in his determination arose from the fact 

 that lead chromate, except when tlirown down from very dilute solu- 

 tions, 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 ])recipitate 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 Peligotf now showed, by his analyses of chromous acetate and 

 of the chlorides of chromium, that the true number was near 52.5. 

 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 considerably, 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 X 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 are those of Berlin, Moberg, Lefort, 

 Wildenstein, Kessler, Siewert, Baubigny, Rawson, and Meineke. 



Among the papers upon the atomic weight under consideration that 

 by Berlin is one of the most important. § His starting point was normal 

 silver chromate ; but in one experiment the dichromate AgaCr.,07 was 

 used. These salts, which are easily obtained in a perfectly pure condi- 

 tion, were reduced in a large flask by means of hydrochloric 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 then collected 

 upon a filter, dried, ignited, and weighed. The results were as follows : 



*Schweigg. Journ., 22, 53, and Poggend. Annal., 8, 22. 



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



t Compt. Rend., 24, 679. 1847. 



t Journ. fiir Prakt. Chem., 37, 509, and 38, 149. 1846. 



