32 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



found by actual admixture to color the fused silver chloride a pale buff, 

 rendering it opaque ; while the latter had no perceptible effect upon its 

 appearance. Thus Stas could not have detected the presence of sodic 

 chloride in his precipitate by the appearance, whether fused or unfused. 

 An impurity of argentic nitrate being thus the most easily detected 

 among the possible impurities, the next step was to use the contamina- 

 tion with this salt as a means of testing the properties of the precipitate. 



It was early found that upon shaking pure argentic chloride with 

 a solution of argentic nitrate, the latter is adsorbed upon the surface of 

 the precipitate. If a solution containing argentic nitrate and the 

 curdy precipitate is divided into two equal volumes, one containing the 

 precipitate and one clear, and if the precipitate is then carefully washed, 

 more argentic nitrate will be found in the portion which contained 

 the precipitate than in the other portion . A further evidence of this 

 adsorption is found in the fact that the theoretical amount of washing is 

 especially inadequate to remove the last traces of adhering electrolyte. 



These experiments show not only that the nitrate is adsorbed, but 

 also that some at least of the adsorbed salt may finally be washed off. 



In view of this result, it seemed probable that the reason why 

 silver chloride occludes silver nitrate in the act of precipitation is 

 because the former adsorbs the latter during this process, and the 

 adsorbed material is then covered up by newly-formed precipitate. 

 The more dilute the precipitating solutions, the less considerable would 

 be the adsorption. 



The peculiar curdy, flocculent form of this precipitate suggested 

 that since the material is so flexible, long-standing and gentle agita- 

 tion with pure water might allow the occluded material to escape, 

 although from a more rigid precipitate this escape is known to be 

 impossible. With this possibility in view, a number of experiments 

 were made in the following manner: Argentic chloride was precipi- 

 tated from several portions of a concentrated solution of the nitrate, 

 thus occluding much (in some cases as much as o.i per cent) of this 

 salt. The effort was then made to wash these precipitates ; and the 

 possibility of thorough washing was found to vary greatly with the 

 treatment of the precipitate. In those cases where the precipitate 

 had remained for two or three days in contact with a mother liquor 

 rich in silver, it was found much more difficult to remove the nitrate 

 than in cases where the washing was more prompt. It seems as if 

 the innermost cells of the precipitate are closed upon standing during 

 the well-known contraction which then occurs. In one case where 

 normal solutions had been used in precipitation, six prompt, gently- 

 agitated washings with pure water were enough to remove practically 



