the Elementary Law of Hydrodiffusion. 489 



solution encompassing the copper electrode rises. It thence 

 follows that galvanic elements which are formed of two un- 

 equally concentrated solutions of copper or zinc sulphate and 

 two copper or zinc electrodes dipping in these solutions, must 

 show an electromotive force of such a direction that the cur- 

 rent produced by it in a closed circuit tends, in consequence 

 of the migration of the ions, to equalize the existing differences 

 of concentration. Observations by Wild and by Lindig, many 

 years since, proved that these electromotive forces really exist, 

 and that the currents springing from them do flow through 

 the element in the direction of increasing concentration. The 

 most recent experiments, made by J. Moser, have completely 

 confirmed the earlier observations. I have repeatedly made 

 extended series of observations for the purpose of ascertaining 

 empirically the connexion between the intensities of these 

 electromotive forces and the concentrations of the salt-solu- 

 tions generating them. They showed that with greater con- 

 centrations the electromotive force between two electrodes of 

 zinc, or two of copper, which are immersed in two solutions 

 of zinc or copper sulphate of different degrees of concentra- 

 tion, is, within a moderate difference of concentration between 

 the two solutions, very approximately proportional to that dif- 

 ference, and that for a very great difference between the con- 

 centrations it can be expressed with perfect precision by a 

 parabolic function of the concentrations of the two liquids. In 

 the publication of my investigations on " an empirical unit of 

 electromotive force, and its absolute value," I will enter more 

 fully into this subject; I here content myself with supporting 

 the correctness of the enunciated proposition by adducing a 

 single series of experiments. 



Four solutions of sulphate of zinc were prepared, containing 

 the following quantities (expressed in grams) of anhydrous 

 salt in the chemical unit of volume — i. e. possessing the fol- 

 lowing concentrations : — 



Solution 1 0-1676 



Solution II 0*2301 



Solution III 0-2858 



Solution IV 0-3213 



A parallelepipedal glass trough was divided into two com- 

 partments by cementing into it a porous earthen diaphragm. 

 By pouring into these compartments any two of the four so- 

 lutions, and putting in two amalgamated zinc electrodes, a 

 galvanic element was produced, the electromotive force of 

 which was measured by Dubois-Reymond's modification of the 

 compensation method. The compensating element employed 



Phil. Mag. S. 5. Vol. 8. No. 51. Dec. 1879. 2L " 



