ATOMIC WEIGHT OF SILICON. 259 



Excess 

 mg. 



With glass bulb 0.04 



0.04 



-0.04 



-0.04 



0.09 



0.08 



Average . 03 



In these experiments' a negative correction of 0.03 mg. was applied 

 to the weight of the glass. 



The filtrate from the glass was then made acid by adding, in the 

 form of 1 normal solution, a quantity of nitric acid equivalent to the 

 sodium hydroxide used. Since a fifty per cent excess of sodium 

 hydroxide was used, and since four ninths was changed to sodium 

 chloride, after the addition of the nitric acid the total acidity was not 

 far from 0.1 normal. This could do no harm, however, for Honig- 

 _schmid 23 has found that even three normal nitric acid liberates no 

 hydrochloric acid or chlorine from a similar quantity of potassium 

 chloride. 



From the corrected weight of silicon tetrachloride the weight of 

 silver necessary to precipitate the chloride was computed. This 

 quantity was weighed out, chiefly in the form of a very few large 

 buttons, the final adjustment being made with small electrolytic 

 crystals. After careful solution of the silver in chloride-free nitric 

 acid and elimination of nitrous acid, in a flask provided with a spray 

 trap in the form of a column of bulbs ground into the neck, the solution 

 was diluted to tenth normal concentration or less, and then was added 

 slowly with constant agitation to the chloride solution contained in a 

 glass stoppered flask or bottle. The mixture was allowed to stand for 

 several days with occasional shaking, before testing for excess of 

 chloride or silver in a nephelometer. If an excess of either was found, 

 the deficiency of the other was made up by adding hundredth normal 

 solution until the endpoint had been reached. Even then the solutions 

 were allowed to stand for some weeks longer with occasional shaking 

 in order to allow included or occluded material to be extracted from 

 the precipitate. Only slight changes were ever produced by this 

 standing. 



The manipulations of precipitation and testing of the solutions were 



23 Honigschmid, Z. Elektrochem., 26, 403 (1920); Ber. d. d. chem. Gesell., 

 54, B, 1873 (1921). 



