Force and Osmotic Pressure. 385 



available to throw light on the questions considered in this 

 paper. The most trustworthy measurements appear to be 

 those of Helmholtz *, made in connexion with the theory 

 already referred to; Wright and Thomson f, MoserJ, 

 Lussana§, and Goodwin ||. Each of these writers, unfortu- 

 nately, expresses the concentration of his solutions differently, 

 making a comparison between their results somewhat difficult. 

 I have, however, chosen all the recorded observations on two 

 salts, zinc sulphate and zinc chloride, and reduced them to a 

 common reckoning by the aid of Kohlrausclr's tables, and 

 supplemented them by fresh experiments. The work of the 

 writers named was mostly on concentration-cells of the usual 

 type, involving migration of ions : it is consequently not 

 directly available for determination of osmotic pressure ; not 

 at present indirectly, since the information available on the 

 transference ratios for these two salts is exceedingly scanty* 

 Such as it is, it goes to confirm the relation indicated by a 

 comparison of equations (2) and (6). On looking through 

 the experimental material, however, a preliminary point 

 attracted attention. Wright and Thomson made measure- 

 ments with cells provided with electrodes of amalgamated 

 zinc, electroplated zinc, and bright polished zinc ; and they 

 found that the electromotive force of a concentration-cell 

 made with the same solutions differed according to the nature 

 of the metal. If this be true, it implies a serious omission 

 in all the theories of the subject that have hitherto been 

 proposed, for however the electromotive force (or solution- 

 pressure) of an electrode may vary with its physical state, if 

 only the two electrodes are alike this variation is eliminated, 

 according to the osmotic theories. The difference found 

 amounted to as much as one third of the total E.M.F. of the 

 concentration-cell : the amalgamated electrodes giving the 

 largest, the polished ones the smallest values ; and similar 

 results were found for copper and cadmium. Notwith- 

 standing this, and the carefulness of Wright and Thomson's 

 work, I am inclined to think that they must have been 

 deceived in the matter, on account of the difficulty in getting 

 concordant results with electrodes that are not amalgamated. 

 I made a test experiment with the materials I had to hand 



* Berl Monatsber. 1877, pp. 713-726 ; Bert. Sitzber. 18S2, pp. 22-39, 

 825-836. 



f Phil. Mas-. [5] xvii. pp. 282-301, 377-391 (1884). 



t Wied. Ann. xiv. pp. 62-8-5 ("1881); Wien. Ber. xcii. pp. 652-656 



(-\*m). 



§ 1st. Venet. [7] iii. pp. 1111-J148 (1892). 

 || Zeits. f.plujs. Chem. xiii. p. 577 (1894;. 



