246 * BAXTER, WEATHERILL AND SCRIPTURE. 



From these figures by the method of least squares Clarke computes 

 the mean, 28.25, but calls attention to the uncertain nature of this 

 value and to the desirability of new determinations of the constant in 

 question. 



In spite of the small percentage of silicon in the chloride and bromide, 

 16.5 and 8.6 respectively, so that the experimental error is magnified 

 six and twelve times in calculating the atomic weight of silicon from 

 the results of the analyses, on the whole these two substances seem to 

 offer marked advantages for the purpose over any other compounds of 

 silicon. Chief of these are the ease both of preparation and purifica- 

 tion by distillation, and of analysis by comparison with silver. 



A brief account of our preliminary experiments with silicon tetrachlo- 

 ride was published not long ago. 10 These experiments point to the 

 value 28.11 for the atomic weight of silicon. Our subsequent experi- 

 ments with both the chloride and bromide confirm the conclusion that 

 silicon possesses an atomic weight lower than has been supposed, and 

 indicate a value 28.06, which is even lower than the result of the pre- 

 liminary experiments. 



Purification of Materials. 



Water. Ordinary distilled water was twice redistilled, once from 

 alkaline permanganate and once from very dilute sulfuric acid. The 

 condensers were block tin tubes, fitted to Pyrex flasks with constricted 

 necks, which served as stills. The connection was made by a water 

 seal, no cork or rubber being used. The water was collected in Jena 

 flasks, generally just previous to use. 



Nitric Acid. Concentrated C. P. nitric acid was distilled through 

 a quartz condenser, the first two-thirds as well as the last tenth being 

 rejected. 



Sodium Hydroxide. The best commercial material was dissolved 

 and the greater part of the carbonate was precipitated with barium 

 hydroxide. The supernatant solution was centrifuged until clear and 

 then evaporated to crystallization in a platinum dish. Centrifugal 

 drainage of the crystals in platinum Gooch crucibles n was followed 

 by rinsing with a small quantity of water and a second drainage. 

 Recrystallization in the same way was continued until the mother 



10 Baxter, Weatherill and Holmes, Jour. Am. Chem. Soc, 42, 1194 (1920). 



11 Baxter, Jour. Am. Chem. Soc, 30, 286 (1908). 



