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the total concentration of the dissolved salt with the effective 

 concentration of the ions, we can write : 



E.M.F. -log^- 1 



where a 2 is the value of a in the solution c 1} and a 2 is the value 

 of a in the solution c 2 . As this expression stands it only 

 allows us to calculate the ratio of a values for two different 

 concentrations of the salt. In order to get absolute values 

 of a we choose some solution of very considerable dilution, 

 say N/1000, and assume that the real dissociation is so large 

 that the value of a given by E.M.F. measurements is identical 

 with the value of 7 given by conductivity measurements. 

 From this starting point it is easy to work out a whole series 

 of a values for different dilutions of the salt. On doing this, 

 however, as will be seen in a later table, the list of a values 

 diverges from the list of 7 values, in the sense that the a 

 values become smaller than the 7 values as the concentration 

 of the electrolyte increases. The term a has been called by 

 G. N. Lewis the " activity coefficient " of the ion and the 

 term (ac) the " chemical activity " of the ion, for it is certainly 

 this term, as measured by E.M.F., which possesses the ther- 

 modynamic significance of chemical activity. Incidentally it 

 may be mentioned that even the employment of the a values in 

 place of the 7 values does not suffice to bring the dissociation of 

 a strong electrolyte into agreement with the Ostwald dilution 

 law, so that the anomaly of strong electrolytes still remains, 

 i.e. a strong electrolyte does not obey the law of mass action, 

 whether conductivity results are correct or not. 



We are therefore faced with two problems — (1) Does the 

 conductivity method give an accurate measure of the actual 

 concentration of the ions ? and (2) Why does the concentra- 

 tion of the ions as determined by conductivity differ from 

 the effective concentration as determined by electromotive 

 force ? In considering these important questions attention 

 must be drawn to a paper by J. H. Ellis entitled " Free Energy 

 of Hydrochloric Acid in Aqueous Solution " (Journ. American 

 Chem. Soc. 38, 737, 191 6), which does not indeed solve the 

 difficulties, but goes a long way towards giving a clear pre- 

 sentation of the position reached at the present time. 



In this summary we shall restrict ourselves to a com- 



