PROGRESS IX PHYSICAL CHEMISTRY. 311 



of finding the solubility of difficultly soluble substances like 

 silver iodide. Goodwin concludes that for solubilities less 

 than o*i x io~ 5 the E.M.F. gives better results than the 

 conductivity. Smale (174) has made experiments to show- 

 that Nernst's theory applies as Ostwald has suggested to 

 gas batteries. The electrodes were partly immersed in the 

 gas and partly in the electrolyte, and the E.M.F was found 

 to be independent of the size of the electrodes and of the 

 nature and concentration of solution, unless in the case of 

 the halogen acids, where secondary reactions are supposed 

 to occur. If hydrogen be put over two solutions of different 

 concentration, the electrode in the stronger solution is 

 positive. In the case of gas chains the E.M.F. and the 

 solution pressure are inversely proportional to the absolute 

 temperature. 



Neumann has determined the potential difference be- 

 tween several metals, including hydrogen, and solutions of 

 their salts. For equal concentrations of the metallic ion 

 the nature of the negative ion had no influence on the 

 E.M.F. On arranging the metals in the order of the 

 potential differences obtained, it was found, as anticipated, 

 that those metals coming after hydrogen, namely, Sb, As, 

 Cu, Hg, Ag, Pb, Pt and Au, were all precipitated by 

 hydrogen from their solutions. The potential differences 

 between platinum and various oxidising and reducing 

 solutions were also obtained, and out of forty cases only 

 four gave positive differences. These were stannous 

 chloride, sodium sulphide, hydroxylamine, and chromous 

 acetate. 



The potential differences between various metals and 

 electrolytes have been measured by Rothmund, who uses 

 a new method (176), and finds that the E.M.F. of a gal- 

 vanic cell is the sum of the above potential differences, and 

 that those between the two metals or the two electrolytes 

 have an inappreciable effect on the E.M.F. 



Streintz (177) has verified Nernst's conclusion that the 

 E.M.F. of a Daniell's cell depends only on the ratio of the 

 concentrations of the solutions of the two sulphates. 



The potential difference between an aqueous solution 



