RICHARDS AND CUSHMAN. — ATOMIC WEIGHT OP NICKEL. 335 



argentic bromide. Since the spongy metal (after slow solution in very 

 dilute nitric acid in a platinum dish) was found to contain no bromine, it 

 is safe to infer that all the soluble impurity had been leached out by the 

 water. This process was repeated with every analysis, the result being 

 always the same. From the weight of argentic bromide thus obtained, 

 the weight of the impurity of sodic bromide was alculated, and wlien 

 this quantity was subtracted from both the original weight of the nickel- 

 ous bromide and the weight of the spongy metal, data suitable for the 

 calcuhxtion of the atomic mass were obtained. 



Iq order to show that tliis method of correcting the results is really 

 exact, it is necessary to prove, first, that no impurity other than a bro- 

 mide remains behind in the nickel, and secondly, that no impurity beside 

 sodic bromide is dissolved by the water. The first point has been already 

 partially considered; we have shown that silica at least was absent.* 

 Since silica was the only non-volatile and insoluble acid likely to have 

 been present, and all the bromine had been dissolved out by water, the 

 only probable impurities were other metals capable of being reduced 

 fiom their bromides by hydrogen. But these, even if they had been 

 present in unsuspected and undiscoverable traces, could have exercised 

 no appreciable effect upon the atomic weight unless their equivalents 

 were widely different from that of nickel. Hence this possibility of error 

 need cause no anxiety. Finally, it has been already stated that no 

 weighable amount of hydrogen was ever found in the metal, hence we 

 are justified in the assumption that the washed out spongy nickel is a 

 safe material upon which to base the calculation of the atomic mass in 

 question. 



The question as to the purity of the sodic bromide in the wash-waters 

 was a matter less easily settled. Careful qualitative and quantitative 

 analysis of the liquid were alone capable of deciding the point, and with 

 only infinitesimal amounts of material such elaborate examination was 

 difficult. To make a very long story short, nothing was found in any of 

 the wash-waters beside sodium, bromine, nickel, and in some of the earlier 

 analyses traces of sulphuric acid. This last impurity may have crept in 

 from the air during the evaporation of the aqueous solutions, or possibly 

 from the towers used for drying the nitrogen and air. In the later analy- 

 ses sulphuric acid was not used in these towers, and was proved to be 

 absent from the nickelous bromide. As its amount was in any ease 

 very small, we felt justified in neglecting it as one neglects an infinitesimal 

 of the second order in the course of mathematical reasoning. 



* See p. 328. 



