242 REPORT— 1897. 



cipitates the metal. Thus Ostwald ' found that the electromotive force 

 of the cell 



Ag I 0-1 AgNOa 1 1-0 KCl | AgCl | Ag^ 



was 0"51 volt. Here the osmotic pressure of the silver ions in the solu- 

 tion of potassium chloride is very small to begin with, and is still further 

 reduced because the solubility of the silver chloride is lowered by the 

 presence of chlorine ions. The pressure can be calculated, and an appli- 

 cation of Nernst's formula leads to a theoretical value for the electro- 

 motive force of 0'52 volt — a remarkable agreement with the observed 

 value. 



Other similar cells, giving high electromotive forces with identical 

 electrodes, were examined by Ostwald. 



Volt. 



1. Silver nitrate (0-1) against silver chloride in potassium chloride . 0'51 



2. „ „ „ ammonia 0-54 



3. „ „ „ silver bromide in potassium bromide . 064 



4. „ „ „ sodiurii thiosulphate . . . .0-84 



5. „ „ „ silver iodide in potassium iodide . . 0'91 



6. „ ,, „ potassium cyanide ... . 131 



7. „ „ „ sodium sulphide 1'38 



Comparisons of other cells, in all cases showing an agreement between 

 the observed values and those calculated on the analogy of Nernst and 

 Planck, will be found in the second volume of Ostwald's ' Lehrbuch,' 

 pp. 848, 850. 



The number of silver ions can also be reduced by adding some sub- 

 stance which, by combining with them, removes them from solution. 

 This is shown by the fact that cells Nos. 2, 4, and 6 in the above list have, 

 like the others, high electromotive forces. 



Other metals have been used as electrodes by Zengelis,** who showed 

 that, in many cases, the electromotive forces of cells whose electrodes 

 were copper, lead, nickel, or cobalt were greater the more the concentra- 

 tion of the ions round one electrode was depressed by the addition of a 

 salt. 



Hittorf "• has even shown that the effect of a cyanide round a copper 

 electrode is so great that copper becomes electropositive towards zinc. 

 Thus the cell 



Cu I KCN I K2SO4 1 ZnS04 | Zn 



furnishes a current which carries copper into solution and deposits zinc. 

 In a similar way, silver could be made positive towards cadmium. 



If we know the concentration of the ions round one electrode, it is 

 possible to calculate them round the other from observations on the 

 electromotive force, and this has been done by Behrend.' 



The same ideas have been applied by Le Blanc '^ to the study of gal- 



' LelirlucTi, 2nd ed. vol. ii. p. 882. 



* In order to prevent the formation of a precipitate an indifferent substance, e.g. 

 KNO,, is interposed between the AgNOj and the KCl. 



' Zeits. physikal. Chem. vol. xii. p. 298 (1893). 

 *■ Zeits. physikal. Chem. vol. x. p. 592 (1892). 



* Zeit$. physilial. Chem. vol. xi. p. 466 (1893). 



« Zeits. physikal. Chem. vol. viii. p. 299 ; vol. xii. 333 ; vol. xiii. 163 (1891-94). 



