162 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



The second, also preliminary series, was made with more dilute solu- 

 tions, and came out as follows : 



In 25 Grammes of Solution. 



Cu. AgBr. 



.26190 1.5478 



.26185 1.5477 



1.5479 



Hence 2AgBr : Ca : : 100 : 16.919, ± .0012. 



In the third series, two distinct lots of crystallized bromide were dis- 

 solved, and the solutions examined in the same way : 



Cu. AgBr. Ratio. 



.2500 1.4771 16.925 



.5473 3.2348 16.919 



Mean, 16.922, ± .0020 



In the final set of analyses, the materials used were purified even more 

 scrupulously than before, and the process was distinctly modified, as 

 regards the determination of the bromine. The solution of the bromide 

 was added to a solution of pure silver in nitric acid, not quite sufficient 

 for complete precipitation. The slight excess of bromine was then 

 determined by titration with a solution containing one gramme of silver 

 to the litre. Thus silver proportional to the copper in the bromide Avas 

 determined, and the silver bromide was weighed in a Gooch crucible as 

 before. The results are subjoined : 



In 50 Grammes of Solution. 



Cu. Ag. AgBr. 



.54755 1.8586 3.2350 



.54750 1.8579 3.2340 



1.8583 3.2348 



Hence Cu : 2Ag : : 100 : 339.392, ± .0108, and 2AgBr : Cu : : 100 : 16.927, 

 ±.0012. 



The latter ratio, combined with the results of the three preceding 

 series, gives a general mean of : 



2AgBr:Cu: : 100: 16.924, ± .0007 



Hence Cu = 63.566. 



In his two earlier papers ' Eichards determined the copper-silver ratio 

 directly — that is, without the weighing of any compound of either metal. 

 By placing pure copper in an ice-cold solution of silver nitrate, metallic 



1 Proc. Amer. Acad., 22, 346, and 23, 177. 1886 and 1887. 



