ATOMIC WEIGHT OF SILICON. 263 



3 and 4, middle fractions from S, 5 and 6, the least volatile material 

 from S, 7, the residue left from the distillation from R and 8, a similar 

 residue from (}. 



The Analysis of Silicon Tetrabromide. 



The analysis of silicon tetrabromide was carried out almost exactly 

 as described in the case of the tetrachloride. The glass of the bulb 

 was washed with one per cent nitric acid and allowance made for re- 

 duction of the weight of the filter ash as described on page 258. When 

 the end point of the comparison had been reached, the portions tested 

 with silver and bromide solutions remained almost perfectly clear. 

 This indicates the absence of appreciable amounts of chloride, and is 

 to be expected in view of the pains taken to remove chlorine from the 

 bromine. 



Weighings were made on a No. 10 Troemner balance, sensitive to 

 0.02 mg. with a load of fifty grams. The beam was graduated to 

 0.05 milligram and the 5 mg. rider was used to determine all quantities 

 larger than this, interpolation from the zero points being employed 

 for amounts less than 0.05 milligram. 



The weights were of gold plated brass, except the fractional weights, 

 and were compared by the Richards 25 method. 



All weighings were by substitution. In the case of the bulb and 

 the silver, the weights were substituted for the object weighed. In the 

 case of the glass, the crucible was substituted for a similar counterpoise. 



Impure radium bromide was kept in the balance case to prevent 

 electrostatic effects. 



Vacuum corrections were applied as follows: 



Humidity was read on an accurate hair hygrometer. 



The analyses of Series I, II, III, and analyses 21, 22, 23, 24 and 25 

 of Series V were performed by Dr. Weatherill. All analyses of Series 

 IV and analyses 26, 27 and 28 of Series V were performed by Mr. 

 Scripture. 



25 T. W. Richards, Jour. Amer. Chem. Soc, 22, 144 (1900). 



