ATOMIC WEIGHTS 



189 



Dumas ' employed barium chloride prepared from pure barium nitrate, 

 and took the extra precaution of fusing the salt at a red heat in a current 

 of dry hydrochloric acid gas. Three series of experiments upon three 

 samples of chloride gave the following results: 



AJ 



C^ 



Aflr. 



BaCh. 



Ratio. 



Mean, 96.316, ± .0055 



Hence Ba = 136.894. 



The work done by Richards " was of a much more elaborate kind, for 

 it involved some collateral investigations as to the effect of heat upon 

 barium chloride, etc. Every precaution was taken to secure the spectro- 

 scopic purity of the material, which was prepared from several sources, 

 and similar care was taken with regard to the silver. For details upon 

 these points the original paper must be consulted. As for the titrations, 

 three methods were adopted, and a special study was made with refer- 

 ence to the accurate determination of the end point; in which particular 

 the investigations of Pelouze, Marignac and Dumas were at fault. In the 

 first series of determinations, silver was added in excess, and the latter 

 was measured with a standard solution of hydrochloric acid. The end 

 point was ascertained by titrating backward and forward with silver 

 solution and acid, and was taken as the mean between the two apparent 

 end points thus observed. The results of this series, with weights reduced 

 to a vacuum standard, were as follows: 



1 Ann. Chem. Pharm., 113, 22. 1860. Ann. Chim. Phvs. (3), 55, 129. 



2 Free. Amer. Acad., 29, 55. 1893. 



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