A TYPICAL DETERMINATION. 41 



to these and similar possibilities. One question especially seemed 

 to demand an answer, namely, that concerning the possibility of the 

 abrasion of powdered glass from the ground-glass stopper of the pre- 

 cipitation flask. 



A newly ground stopper sheds glass noticeably. It must always 

 be polished and rubbed a long time until the loose particles are thor- 

 oughly dislodged. A flask of soft glass never ceases to grind away 

 slightly every time the stopper is inserted. The large 2 -liter Erlen- 

 meyer flasks used in the final experiments were of very hard insoluble 

 glass, with rather wide necks, very suitable for precipitation and for 

 washing. In order to test the extent of their abrasion, one of the 

 stoppers was removed and replaced a hundred times. The resulting 

 powdered glass was collected on a small fine filter and found to weigh 

 0.2 mg. Since, during a single determination, the stopper was not 

 replaced over a dozen times, it is safe to infer that the error from this 

 cause could not have exceeded 0.02 mg. a negligible quantity. 



Other sources of contamination were so carefully guarded against 

 that they can hardly have been present. 



The foregoing general description may be profitably illustrated 

 by recounting the details of a single experiment, before the results of 

 all the experiments are given. 



Experiment 63. 



April 5, 1904. Salt J. 



Grams. 



Corrected excess weight of crucible over counterpoise 0.19994 



Corrected excess crucible plus fused salt over counterpoise.. . 576754 



Weight salt in air (24 , 764 mm.) 5.56760 



Vacuum correction -f- 0.00231 



Weight of sodic chloride in vacuum 556991 



As a suitable excess of silver nitrate, 16.40 grams were taken for 

 precipitation. The concentration of the silver salt being about fifth 

 normal and that of the sodic salt not much greater, the total mother 

 liquor was 1 .4 liters. Five washings of the precipitate were made after 

 it had stood over night, with 0.2-liter portions of water containing 

 about 0.05 gram of silver nitrate per liter. Washings Nos. 6 and 7 

 were made with pure water and collected separately. Washings 8 to 

 12 were made with slightly acidified water and collected separately. 

 The whole precipitate was finally washed upon the Gooch crucible. 

 After drying it appeared perfectly white. 



