430 Dr. R. A. Lebfeldt on the Theory of 



The optical principle involved in this compound prism, of 

 obtaining a comparison viewing-screen with vanishing line 

 in the dispersive prism itself, may be extended to other forms 

 of instruments, such, for example, as Helmholtz's colour- 

 mixing apparatus with two collimating telescopes containing 

 Rochon prisms. Experimental results of this nature are 

 reserved for future publication. 



University of Nebraska, 

 July 29, 1899. 



XLVI. On the Theory of the Electrolytic Solution-Pressure. 

 By R. A. Lehfeldt, D.Sc* 



rt ^HE electromotive force between a metal and an electrolyte 

 JL depends on the concentration, in the latter, of the ions 

 of the same kind as the former ; and it is easy to show in 

 certain cases, on an irreproachable basis of thermodynamic 

 reasoning, that variation of the electromotive force is propor- 

 tional to that of the logarithm of the concentration. Thus, 

 supposing the electrolyte to be a binary univalent salt 

 (e. g. KCi) and completely dissociated, then if C be its con- 

 centration (gm. mol. per cub. centim.), the E.M.F. of the 

 contact may be expressed in the form 



RT 

 E = log e C + const., 



where R= gas constant, T= temperature, e= quantity of 

 electricity associated with one gram equivalent : or, if we 

 prefer to use instead of C the osmotic pressure P produced 

 by the metallic ions, 



E= log e P + const., 



where the only difference is in the value of the constant. So 

 far no assumptions are required except those usually implied 

 in the thermodynamic treatment of solutions with the aid of 

 semipermeable membranes. 



form here shown. Certificates of calibration might be furnished with 

 the instrument, but it should be borne in mind that the curve of luminosity 

 depends on subjective conditions, and a calibration by one individual 

 might differ from that of another, particularly if the eye of one is 

 trichromatic and that of the other dichromatic or monochromatic. How- 

 ever, the calibration curves of different observers have been found to be 

 the same, which agrees with the results of Konig, who has shown that 

 the slope of the curve is approximately the same in these different cases. 



* Communicated by the Author : read before the British Association, 

 Sept. 20, 1899. 



