a revision of the atomic weights of sodium 



and Chlorine. 



By Theodore William Richards and Roger Clark Wells. 



INTRODUCTION. 



The investigation of the atomic weights of the extremely common 

 elements sodium and chlorine was undertaken because of an unac- 

 countable discrepancy which appeared in the composition of a sample 

 of very pure sodic bromide, as compared with the results of Stas. 

 This sodic bromide had been prepared for the purpose of determining 

 the transition temperature of its hydrated crystals ; and a preparation 

 which yielded a constant transition point, therefore giving evidence of 

 great purity, nevertheless possessed a perceptibly lower combining 

 weight than that indicated by Stas's results. 



Such a discrepancy as this was not to be passed over lightly. It 

 indicated either an unknown constant impurity in our sodic bromide 

 and hence a possible error in our transition temperature or else a 

 flaw in the classical work of Stas. When the first cause of disagree- 

 ment had been carefully sought in vain, the second was pursued. 



Thus a physico-chemical investigation demanding great purity 

 of materials led to a quantitative research of unexpected magnitude ; 

 and in turn this quantitative investigation depended continually upon 

 physico-chemical methods and considerations. In confirmation of 

 previous work at Harvard, it was found that the physico-chemical 

 conditions of experiment were of as serious import as the purity of the 

 materials, and of far greater significance than an increase in the scale 

 of operations. The essential circumstances here defined must receive 

 consideration in any other research of a similar kind. 



It is needless to say that an investigation seeking to test the ac- 

 curacy of Stas's work is a serious undertaking, for undoubtedly much 

 of his work deserves a rank among the most accurate of quantitative 

 chemical results ; certainly he distanced all those who preceded him. 



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