ATOMIC WEIGHT OF SILICON. 267 



the results of the analyses of Sample IV should be given preference to 

 those of the first three samples of chloride. 



The silicon tetrabromide, although distilled at the beginning of the 

 fractional distillation without the use of fractionating columns, ulti- 

 mately was repeatedly distilled in much the same way as Sample IV of 

 the chloride. Here also uniformity in composition over a wide range 

 of fractions was secured. The fact that the least volatile fraction, 

 No. 8, gave a distinctly higher result than the remainder, is counter- 

 balanced by the fact that the three residual fractions from distillations 

 immediately following, were apparently no different from the more 

 volatile fractions. 



Furthermore, -the fourth chloride series and the bromide series 

 yielded almost identical results. Therefore the average of the mean 

 values from these two series, 28.063, is the most probable value which 

 can be derived from these data. It seems unlikely that further 

 fractional distillation would have affected this outcome. 



It is interesting to compare the influence which various impurities 

 would have on the results. In the following table are given the effects 

 produced upon the apparent atomic weight of silicon by one-tenth of 

 one per cent of the more likely impurities. 



The bearing of the outcome of the foregoing work upon the isotopic 

 character of silicon is an interesting one. Aston 26 finds ample evi- 

 dence of the existence of two isotopes of this element, with masses 

 28 and 29. He then continues: 



"The evidence of a silicon of atomic weight 30 is of a much more 

 doubtful character. Its presence is suggested by the lines 30, 49, 68 

 and 87, but the possibility of hydrogen compounds makes the evidence 

 somewhat untrustworthy, and no proof can be drawn from a second 

 order line 15, as this is normally present and is due to CH 3 . On the 

 other hand, if we accept a mean atomic weight as high as 28.3, the 

 relative intensity of the lines due to compounds of Si 28 and Si 29 indi- 

 cates the probable presence of an isotope of higher mass." 



26 Phil. Mag., 40, 628 (1920). 



