Double-Layer of Solid and Liquid Bodies. 313 



theory with experiment may really be effected, is Pt, and 

 it happens that the calculated value of V exactly agrees 

 with that derived under nearly the same conditions from the 

 thermionic effect*. In fact, the latter has been measured 

 for very high temperatures, as well ns the surface-tension of 

 fused platinum, after an almost complete expulsion of 

 occluded gases, and is really characteristic of pure platinum. 

 This cannot be said about Na, no care having been taken in 

 Millikan's experiments to get rid of occluded gases. There 

 are strong indications to the effect that the latter greatly 

 diminish the intrinsic potential of the alkali metals t, and it 

 is very probable that the calculated value of V (4/2 volts) 

 for pure Na is nearer to the truth than Millikan's value. 



The exceedingly high fusion-point of the other elements 

 in Table II. has not allowed hitherto of a determination of 

 their surface-tension. Their intrinsic potentials, however, 

 are very close to those calculated in Table III., so that at 

 first sight it may seem that both are got from the same 

 source. 



Further, the value of V for cadmium seems too high and 

 that for Zn too low. This may perhaps be due to a corre- 

 sponding error in the experimental values of <r; it is, indeed, 

 very strange that the surface-tension of metals so closely 

 akin as zinc and cadmium should be so widely different. 

 Whatever it may be, the Table correctly expresses the fact 

 that Zn is, under ordinary conditions, one of the most electro- 

 positive metals, highly photo-electric, and positively electrified 

 when put in contact with all the other metals except the 

 alkali ones. More than that, if we arrange the elements of 

 Table III. (except Na, K, and Cd) in the order of increasing 

 intrinsic potentials, we get the following series : 



( + ) Zn, Sb, Bi, Fe, Pb, Hg, Sn, Pt, Ag(-), 



every member of which should be positively electrified when 

 put in contact with one of the next. This series (corre- 

 sponding to the " Volta series ") is not very different from 

 those which were experimentally found. For instance. 

 Volta found: ( + ) Zn, Pb, Sn, Fe, Ag (- ) : Seebeck : 

 ( + ) Zn, Pb, Sn, Sb, Bi, Fe, Pt, Ag (-) ; Auerbach : 

 ( + ) Zn, Sn, Pb, Sb, Bi, Hg, Fe, Ag, Pt (-). 



Lastly, the theoretical values of the contact electromotive 

 forces, equal to the difference between the calculated 



* See Table II., § 4. _ 



t The repeated distillation of potassium in a very high vacuum 

 caused its photo-electric effect to disappear completely. See Hughes, 

 Phil. Mag. vol. xxviii. p. 338 (1914). Also Wiedemann & Hallwachs. 

 Verh. d. Deutsch. Phi/s. Ges. p.. 107 (1914). 



Phil. Mag. S. 6. Vol. 33. No. 196. April 1917. Y 



