76 PROCEEDINGS OF THE AMERICAN ACADEMY. 



As was to have been expected, the impurity is greater here than in 

 Series II., III., and VI., but less than in Series IV. and V. 



It was not the intention of this paper to extend the investigation to 

 the study of the occlusion of other chlorides, which must necessarily 

 complicate the problem greatly. It is very interesting to note that the 

 amount of occlusion seems to be due rather to the amount of chlorine 

 present than to the amount of barium, for hydrochloric acid increased 

 the occlusion almost as much as an equivalent amount of baric chloride 

 did. This observation leads one to conclude that a careful study of 

 the phenomenon from a physico-chemical point of view might furnish 

 some satisfactory clue as to the nature of such occlusion in general ; 

 and it is hoped that before long a collection of suitable data upon the 

 problem may be obtained here.* The present paper deals only with 

 the solving of an analytical problem. 



At first sight, the facts shown by this paper appear to be contradic- 

 tory to those stated by F. W. Mar,f who recommends the use of strong 

 nitric and hydrochloric acids for the precipitation. It must be borne 

 in mind, however, that Mar was precipitating the barium with an 

 excess of sulphuric acid, and hence that a considerable amount of 

 occlusion could make very little difference in his case. When one is 

 determining sulphuric acid by means of an excess of barium, nothing 

 could be worse than the addition of a large amount of hydrochloric 

 acid ; and this point cannot be too strongly emphasized. 



Mar rightly states that a considerable excess of sulphuric acid is 

 necessary to secure rapid precipitation of all the barium in strongly 

 acid solutions. If Fresenius had realized that an excess of a common 

 ion in solution decreases the solubility of a precipitate, he would not 

 have confounded the solubility of baric sulphate in a strong solution of 

 baric chloride with its solubility in pure water, J or in water contain- 

 ing free hydrochloric acid. 



The conclusions reached in the present paper are as follows : — 



First, that the occlusion of baric chloride by baric sulphate may lead 

 to very serious error. 



Second, that the amount of this occlusion is greater in concentrated 

 than in dilute solutions, greater in the presence of hydrochloric acid 

 than in its absence, and greater when the sulphate is poured into the 



* For the treatment of an analogous case, see E. A. Schneider, Zeitschr. 

 Phye. Chem., X. 425. 



t Am. J. Sci., [3.], XLI. 288, XLV. 399. 

 } Zeitschr. f. Anal. Chem., IX. 52. 



