68 PROCEEDINGS OF THE AMERICAN ACADEMY. 



found in the very decided occlusion of baric chloride by baric sulphate, 

 a fact which has already been discussed in a previous paper.* This 

 cause of error was never wholly avoided, although by repeated treat- 

 ment of the baric sulphate with sulphuric acid, all but a few tenths of 

 a milligram of chlorine were found to be expelled. Whether or not 

 the baric chloride is occluded in the form of a so called " solid solu- 

 tion," as E. A. Schneider suggests in a recent paper with regard to 

 the occlusion of ferric sulphate by baric sulphate, f the fact remains 

 that an appreciable amount of baric chloride is very firmly held. 

 Possibly nothing short of the true solution of all the baric sulphate in 

 sulphuric acid could drive off all the chlorine ; and such treatment 

 would be likely to introduce errors as large as the one which it would 

 obviate. 



Besides this inclusion of baric chloride, which tends to raise the 

 apparent atomic weight of barium, another circumstance, the solu- 

 bility of baric sulphate in water, may work in the same direction. 

 This second cause of error was quite eliminated in the present work. 

 A third cause of error with an opposite tendency exists; namely, the 

 obstinacy with which baric sulphate retains the last traces of the ex- 

 cess of sulphuric acid and water used to precipitate it. It must be 

 borne in mind that the retention of 0.2 milligram of either substance 

 counterbalances numerically the retention of nearly two milligrams of 

 baric chloride. 



Two series of determinations were made, one by the direct addi- 

 tion of pure sulphuric acid to a concentrated solution of pure baric 

 chloride in a platinum crucible, the other by precipitation in dilute 

 solution after the usual fashion. In the latter case the baric sulphate 

 in the filtrates, usually amounting to about a milligram and a half, 

 was determined by evaporation in large platinum dishes. In each 

 series every specimen of baric sulphate was of course heated with 

 successive drops of pure sulphuric acid at a dull red heat until the 

 weight became constant. Eight experiments which were not compli- 

 cated with mechanical errors are recorded in the note-book. The 

 two series gave the same result, a hundred parts of baric chloride 

 yielding 112.073 parts of baric sulphate. The highest result was 

 112.087, and the lowest 112.060. Berzelius found 112.17.3, Turner 

 112.19, Thomson about 112.15, Struve 112.094, and Marignac 

 112.011. 



* These Proceedings, XXVI. 258. 



t Zeitschr. fur Physikal. Chem. (1892), X. 425. 



