BAXTER. — ATOMIC WEIGHT OF CHROMIUM. 403 



sible merely to indicate the nature of the difficulties ; as a rule it is 

 impossible to estimate the magnitude of the error without repetition 

 of the experimental work. Hence in this paper attention is called 

 only to points in the earlier work which have been experimentally 

 investigated. The uncertainty in most of the previous determinations 

 is emphasized by the lack of agreement in the individual analyses in 

 each series, as well as in the different series. 



The choice of method for this investigation was influenced by several 

 considerations. In the first place, the substance to be analyzed must 

 be definite, in composition and capable of being either fused or heated 

 to a high temperature in order to insure the elimination of moisture. 

 In the second place, in view of the fact that chromium is hard to 

 handle satisfactorily in a quantitative fashion, the analytical operation 

 should involve the determination of some other element. The halogen 

 compounds, which have been employed very successfully many times, 

 especially in this laboratory, for the determination of the atomic 

 weights of metallic elements, are less suited for use in the case of 

 chromium on account of the difficulty in the complete precipitation 

 of the halogens by means of silver nitrate. All things considered, the 

 chromates of silver seemed to offer the most promising possibilities on 

 account of the ease with which their silver content may be determined. 

 It is true, in order to determine the ratio of the atomic weight of 

 chromium to that of either silver or oxygen, this method necessitates a 

 knowledge of the exact ratio of the atomic weights of silver and 

 oxygen, knowledge which is at present lacking. The per cent of 

 silver in the compound being known, however, analytical data may be 

 used at any subsequent time for the calculation of the atomic weight 

 of chromium. Furthermore, since the value for the atomic weight of 

 chromium at present accepted depends very largely upon the analysis 

 of silver chromate, a study of this salt with the application of the most 

 modern methods seemed to promise interesting results, and therefore 

 was first taken up. In a following paper is given a description of the 

 analysis of silver dichromate. 



Purification of Materials. 



Water. — The laboratory distilled water was twice redistilled, once 

 from alkaline permanganate and once from very dilute sulphuric acid. 

 In both distillations block tin condensers were employed, no cork or 

 rubber connections being necessary. 



Silver Nitrate. — The preparation of pure neutral silver nitrate for 

 the precipitation of silver chromate followed the lines laid down in pre- 



