OCCLUSION OF DISSOLVED SUBSTANCES. 33 



all the occluded material and yield a chloride which fused without 

 blemish ; but this praiseworthy behavior was the exception rather 

 than the rule. In this case the innermost cells must have been cleaned 

 before they closed during the compacter aggregation of the precipi- 

 tate ; but usually traces of silver nitrate were inclosed, and when they 

 were once inclosed were beyond the reach of even a dozen washings. 

 Therefore more dilute solutions of argentic nitrate were used in 

 precipitating, in order to find the limit beyond which a satisfactory 

 precipitate was certain. Third normal solutions were found to be 

 still slightly too concentrated, but fifth normal solutions always 

 3'ielded a satisfactory result, if speedily but gently washed six times, 

 and then occasionally agitated in pure water in order to dissolve out 

 the last trace of nitrate. In previous determinations in this laboratory 

 still less concentrated solutions (tenth normal) were used, so that no 

 correction need be applied to those determinations on this account.* 

 This further dilution is necessary unless the curdy precipitate is 

 speedily washed or agitated with a solution containing no excess of 

 silver. 



Having thus obtained light on the mechanism of the occlusion of 

 argentic nitrate, we turned our attention to the possibility of occluding 

 sodic chloride. Here, as had been said, no qualitative experiment can 

 conveniently prove the presence of the impurity. Hence recourse was 

 had to a quantitative experiment. An amount of pure salt known by 

 previous experiment with very dilute solutions to be exactly equivalent 

 to a given weight of pure silver was carefully weighed out, and the 

 solid was dropped into a fairly concentrated solution of the nitrate, 

 according to Stas's later method of procedure. After two hours' 

 thorough shaking and a day's standing, which Stas had always given, 

 it was found that 0.014 P er cent more of salt had to be added in order 

 to attain exact equivalence, showing that this amount of salt remained 

 occluded by the precipitate even after the shaking and standing. Agi- 

 tation for another day leached out about one-half of the occluded salt, 

 but seven days' soaking and agitation were required before the full 

 amount (0.014 P er cent) of extra silver nitrate was needed to attain 

 equivalence with the salt dissolved out of the precipitate. Subse- 

 quently in several days no more salt appeared, hence the final condi- 

 tion must have been reached. Since probably some silver nitrate 

 also was occluded, it is clear that these figures represent a minimum 

 value, and the certain error introduced by Stas's method of procedure 



*See for example, Richards, Proc. Am. Acad., 29, 76 (1893). 



