OF ARTS AND SCIENCES. 195 



XII. 



CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE. 



THE ANALYSIS OF CUPRIC BROMIDE, AND THE 

 ATOMIC WEIGHT OF COPPER. 



By Theodore William Richards. 



Presented by the Corresponding Secretary, October 8, 1890. 



Introduction. 



The uncertainty of knowledge regarding the atomic weight of copper, 

 owing to the widely different values assigned by various experiment- 

 ers, led nearly four years ago to the beginning of an investigation, of 

 which this paper is the third publication. The two earlier papers * 

 described a number of determinations made by a method which had 

 not until then been successfully applied, — the precipitation of metallic 

 silver from neutral silver solutions by means of metallic copper. The 

 results obtained were in themselves very satisfactory, but differed en- 

 tirely from the value which has hitherto been accepted with little ques- 

 tion by chemists ; and the desire to discover, if possible, the cause of 

 this discrepancy, has finally led to the continuance of the investigation. 



It is well known that we can rarely find with regard to any element 

 more than one or two compounds which answer the exacting require- 

 ments of atomic weight determination ; although others of its definite 

 compounds may by their analysis more or less strongly support the 

 verdict of the more exact results. Disagreement imjjlies constant 

 error in one or other of the disagreeing values, and the detection and 

 correction of this constant error can alone give certainty to our con- 

 clusions. Much, however, may be learnt by careful study from a 

 new standpoint, and the first step in the present investigation was to 

 discover if possible such an opening. 



The basic tendency of cupric salts, and the ready oxidation of 

 cuprous compounds, complicate to such an extent the relations of 

 copper, that the preparation of any one of its salts in a dry state 



* These Proceedings, xxii. 342, and xxiii. 177 ; also Fresenius's Zeitschrif t, 

 xxviii. 392. 



