268 



BAXTER, WEATHERILL AND .SCRIPTURE. 



It is obvious that the atomic weight derived in this paper is more 

 nearly in accord with the evidence yielded by mass spectra as regards 

 the proportion of the isotopic components of silicon, than the value 

 28.3 which has been in general use for some time. Assuming the 

 existence of appreciable proportions of only the two isotopes 28 and 29, 

 our result indicates that the ratio of lighter to heavier is about 14 to 1. 



^Yith at least two isotopes of silicon, chlorine, and bromine, there 

 are evidently possible ten different tetrachlorides of silicon as well as 

 ten different tetrabromides, with molecular weights ranging from 168, 

 at intervals of one unit, to 177 in the case of the chloride, and from 

 344 to 353 in the case of the bromide. Of these, the two lightest, 

 Si 28 (CI 35 ) 4 , 168, and Si 28 (Br 79 ) 4 , 354, are presumably the most abun- 

 dant and the most volatile. 



The question therefore may be raised as to whether partial separa- 

 tion may have occurred during the fractionation of the two liquids. 



If, however, separation actually occurs at all during the fractiona- 

 tion, it certainly is to be expected that it will continue to take place 

 during the whole fractionation. The absence of any systematic 

 difference in composition among the fractions analyzed in the case of 

 Series IV and V, which covered a large proportion of the original 

 material, is strong evidence that no important separation actually 

 was taking place. 



This is only to be expected from the fact that the conditions of dis- 

 tillation were far from ideal for the separation of isotopic substances, 

 for as Mulliken and Harkins 27 and others have pointed out, such a 

 separation is to be expected only when distillation takes place at ex- 

 tremely low pressures. 



We are very greatly indebted to the Wolcott Gibbs and Bache Funds 

 of the National Academy of Sciences for generous assistance in pro- 

 viding the necessary apparatus and materials. 



Summary. 



1. Improved methods for fractional distillation out of contact 

 with the air at various temperatures have been devised. 



2. The analysis of pure silicon tetrachloride and tetrabromide has 

 yielded the values 28.067 and 28.059 for the atomic weight of silicon. 

 The average value is 28.063 (Ag = 107.SS0). 



27 Jour. Amer. Chem. Soc, 44, 143 (1922). 



