68 PROCEEDINGS OP THE AMERICAN ACADEMY. 



much as possible the error in question. It is also generally known 

 that when the precipitate cakes together upon ignition, the occlusion 

 has been large, and the result will usually be too high. But no sys- 

 tematic series of determinations of the conditions of experiment, 

 and no exact method for wholly eliminating the error, have been de- 

 vised and executed ; hence the present work was undertaken with the 

 idea of solving these difficulties. 



In the course of an extended series of experiments upon the atomic 

 weight of copper* finished several years ago, the attempt was made 

 to determine the sulphuric acid in cupric sulphate by means of the 

 usual analytical method. In order to correct the result for the occlu- 

 sion of baric chloride, the precipitate, after having been weighed, was 

 fused in pure sodic carbonate ; and the chlorine in the aqueous solu- 

 tion of the residual cake was determined, calculated as baric chloride, 

 and subtracted from the total weight of the precipitate. 



The result in question was still vitiated by two opposite errors, the 

 solubility of baric sulphate, and an incorrect atomic weight of barium, 

 and hence it was rejected at the time. Nevertheless, the method 

 seemed to be of value, and the stated intention of investigating it more 

 closely has at last been carried out. 



The objects of the present work were as follows : — 



First, to discover if an accurate determination of sulphuric acid 

 may be made by substracting from the weight of the baric sulphate 

 that of the baric chloride found in it ; and 



Second, to define the conditions which determine the amount of 

 the occlusion, and thus to show how the error may best be avoided 

 altogether. 



A standard solution of pure sulphuric acid (containing 3.2 per cent 

 of H2SO4) was used as the test substance, being weighed in tightly 

 stoppered light glass bottles. The baric chloride used for its precipi- 

 tation had been purified by repeated crystallization, and made up into 

 another standard solution, which was used in appropriate quantities. 

 The sodic carbonate was also recrystallized many times ; and this sub- 

 stance, as well as the nitric acid and all other substances used in the 

 chlorine determination, were proved to be free from the halogen. 



The platinized brass weights used were standardized with great 

 care, and the weighings were corrected to the vacuum standard. Due 

 allowance was made in each case for the weight of the filter ash, and 

 every precaution usual in ordinary analytical work was taken to in- 



* These Proceedings, XXVI. 258. 



