380 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



the precipitate or deposited upon it even after the precipitation of the 

 calcium salt. 



This points to another flaw in the earlier work, — namely, the solutions, 

 which had all been allowed to stand for at least sixteen hours before 

 filtering, had been left too long in contact with their precipitates. 



In a case of this kind great accuracy is to be obtained only by a succes- 

 sion of approximations ; hence it seemed worth while again to make a 

 series of precise analyses, embodying all the advantages which had been 

 found up to this point, in order to discover from their possible variations 

 if there might be still another cause of error as yet undetected. 



Newly prepared very pure solutions were used in these analyses, 

 which were made two years after the ones previously detailed. All the 

 precautions suggested by the foregoing pages were heeded, and two 

 series of results were obtained, one from calcium solutions only, and the 

 other from solutions containing precisely the same amount of calcium with 

 an equivalent amount of magnesium. These are given below in parallel 

 columns : — 



Precipitation of Pure Calcic 



I ».\ALATE. 



(Volume at precipitation = 200 c.c.) 



Precipitation of Calcium in 



Presence of Magnesium. 



(Volume at precipitation = 500 c.c.) 



The agreement between the two averages is very striking; it is to 

 some extent due to a compensation of errors. Even when we have 

 made due allowance for these, however, it furnishes Btrong evidence in 

 favor of the hypothesis which led to the before mentioned precautions! 

 Experiments :.".* and •">". not included in the average, show that at the 



