112 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



TABLE XIII. 



metals Tig;, Zn, and Cd in solutions of chlorides, bromides, and iodides, 

 while in Table XII. are the corresponding values for the single liquid, 

 non-reversible cells with Zn and llg, Cd and Hg, as electrodes.* 



We notice that the numerical change in passing from a chloride to 

 a bromide or iodide solution is the same for these three metals and that 

 the sign is the same for zinc and cadmium, as is shown in Table XIlI. 

 This enables us to formulate matters a little more clearly. The poten- 

 tial difference between a metal and a salt solution is the sum of two 

 terms, one due to the metal and the solvent, the other to the negative 

 ion. For certain metals in certain solutions the term due to the nega- 

 tive ion is independent, numerically, of the nature of the metal con- 

 sidered. For instance, the potential differences ZnlKCl, ZnlKBr, 

 CdlKCl, CdlKBr, IlglKCl, and HglKBr will be A i- a, A + b, 

 Ji -{- a, B -{- b, C—a, and C — b. The electromotive forces of the 

 cells ZnlKCllCd and Zn|KBr|Cd will be A\ = J + « — ^— «, and 

 ^2 = -4 + b — B—b, whence we see that E^ — E^, which had already 

 been found experimentally. For ZnlKCllHg and ZnlKBrlHg we 

 shall have E]_ = A-\-a — C-f- a and En^ — A-\-b — C-{-b, and Ey will 

 not be equal to E.^.- ^y referring to Table III. we can now extend 

 our generalization and make it more precise. With the metals Mg, 

 Zn, Cd, Sn, Pb, and Bi in solutions of chlorides, bromides, iodides, sul- 

 phates, nitrates, acetates, carbonates, and oxalates, the term due to the 

 negative ion is not a function of the electrode. There is not much 

 doubt that the alkaline metals, the metals of the alkaline earths, and 

 the metals of the iron group belong in this same series. Ostwald's 

 measurements show that most organic acids may be added to the above 

 list of solutions. With mercury the numerical value of the term due 

 to the negative ion is the same as with the previous metals, but the 

 sign is opposite. With platinum the numerical value is no longer 



* There is certainly an error in the relative positions of Sn and Pb as given 

 by my determinations, so I give no data for them in Table XII. 



