46 THE ATOMIC WEIGHTS OF 



of fluorine. The work of Me Adam and Smith, 12 who converted 

 sodium fluoride into sodium chloride, is the only modern deter- 

 mination on record which seeks to establish such a direct ratio 

 between these two halides by chemical means. Unfortunately, 

 their work was interrupted prematurely and the experience of 

 those authors shows that the reaction in question did not proceed 

 smoothly. 



It would seem that the ratio of fluorine to another halogen, 

 say chlorine, may be established much more readily by an indirect 

 comparison that is to say, by converting a sample of a suitable 

 compound to the chloride, and another sample of the same prep- 

 aration to the fluoride by an independent analysis. The ratio 

 between the two halides could then be easily calculated. In the 

 present work borax has been used as such an intermediate sub- 

 stance for the indirect comparison of sodium chloride and sodium 

 fluoride. Originally it was intended to repeat the conversion of 

 borax into sodium chloride with larger quantities of material, for 

 in experiment iv the quantities involved are rather small, but 

 this plan was abandoned in favor of the conversion of borax into 

 the other salts. At any rate, even the single conversion to the 

 chloride aids in exhibiting the applicability of the indirect method 

 in this particular case. 



Such a comparison, by cross-reference, has certain general 

 advantages over a direct comparison. In the determination of an 

 atomic weight, from a direct ratio, the initial substance must 

 not only be pure in the ordinary sense, but its composition must 

 exactly correspond to the formula assumed. In an indirect com- 

 parison, the initial substance must also be free from impurities, 

 it is true, but the exact proportion of the elements within the 

 compound is immaterial. In the present case the borax should 

 consist of a union of pure Na 2 O and pure B;0 3 ; the exact ratio 

 in which these two oxides are combined is of no consequence so 

 long as we draw upon the same homogeneous preparation for 

 analysis. Obviously the atomic weight of boron need not be 

 known in this case. 



The new value for fluorine (19.005) is considerably lower than 

 that calculated by Clarke (19.041) 13 and also lower than that 



12 J. Am. Chem. Soc. 34, 592 (1912). 

 "Op cit., p. 408 (1910). 



