COPPER. 97 



The second, also preliminaiy 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 2 AgBr : Cu : : 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, zh .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 was 

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

 before. The results are sul)joined : 



In 50 Grammes of Solution. 

 Cu, Ag. As: Br. 



•54755 i.»5»6 3-2350 



•54750 '1-8579 3-2340 



1.8583 3-2348 



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

 ± .0012. 



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

 gives a general mean of: 



2 AgBr : Cu : : lOO : 16.924, ± .0007 



In his two earlier papers * Richards determined the copper-silver ratio 

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

 By i)lacing pure copper in an ice-cold solution of silver nitrate, metallic 

 silver is thrown down, and the weights of the two metals were in equiv- 



*Proc. Anier. Acad., 22, 346, and 23, 177. 1SS6 and 1887. 



