384 Dr. R. A. Lehfeldt on Electromotive 



is such as to make it practically certain that large deviations 

 from Boyle's law occur. This alone would of course render the 

 usual logarithmic formula largely incorrect ; it would have the 

 same effect on the formula (2) developed above for the electro- 

 motive force of concentration-cells. But if that formula is 

 correctly deduced from the assumptions made, a comparison 

 between the values derived from it and the observed electro- 

 motive forces will afford a measure of how far the osmotic 

 pressure deviates from that indicated by Boyle's law. 



Experiments have therefore been undertaken in order to 

 carry out this comparison. But in the equation to be used, 

 another quantity occurs which is open to great uncertainty: 

 this is the transference ratio x. It is well known that for 

 concentrated solutions the value of x departs very consider- 

 ably from the fixed value it possesses in dilute solutions, 

 and as both measurement of it is difficult and the interpre- 

 tation of the measurements is dubious, it is highly desirable 

 to avoid it if possible. The way to do so was long ago 

 shown by Helmholtz, and carried into practice by him and 

 by Moser. It consists in breaking up the concentration-cell 

 and interposing a pair of similar " non-polarizable " elec- 

 trodes. Thus whilst in the cell 



Zn : ZnSQ 4 (dil.) : ZnS0 4 (cone.) : Zn 



migration of the ions occurs, in the couple 



Zn ; ZnS0 4 (dil.) : Hg s S0 4 : Hg :Hg : Hg 2 S0 4 : ZnS0 4 (cone.) : Zn 



it is avoided. Consideration of a thermodynamic cycle appli- 

 cable to the latter, but otherwise similar to that described in 

 § i., shows that the electromotive force of the couple is equal 

 to that of the simple concentration-cell divided by 1— x, or 



E=^[(»-l)(72-7 1 ) + J c C! {l+("-l)7}^] 



(<9 



In the tables given below are values calculated from this 

 formula by means of Kohlrau sen's tables of conductivity, and 

 the values of y deduced from them : they involve, of course, 

 the assumption that the degree of dissociation y is really that 

 calculated from the molecular conductivity. This assumption 

 is possibly not quite correct, but it is certainly the best we 

 can at present make. 



(vL) Previous Measurements. 



A considerable number of experiments on concentration- 

 cells have been published, but only a part of these are 



