-8 



Theory of Galvanic Polarization. 431 



centim. centim. 



1 

 Contact electricity 1 x 1CT 8 4XIO" 8 



Surface-tension ^ x 10~ 8 



Kinetic theory of gases _xl0~ 8 



7 1 



Solids and liquids ^ X 10 8 c x 10 



to which we may now add 



- Helmholtz 1 x 1(T 8 ; 



3 



while Lippmann's method places the mean at ^x 10" 8 



centim. ; and the other method here given places it at 



4 8 



yv x 10 centim. , with as small limits of error as any of the 



methods given above. 



8. The chief value of this discussion seems, however, to be 

 not so much that it gives an estimate of molecular distance, 

 but that its very close agreement with the other independent 

 estimates derived from considerations connected with the same 

 phenomenon of galvanic polarization is strong evidence of the 

 substantial and ultimate truth of that representation of the 

 phenomenon which has formed the basis of the discussion. 



This argument seems to derive very great weight from the 

 wide variety and very different magnitudes of the physical 

 constants employed in the three calculations, one depending 

 on the direct measurement of the polarizing charge, another 

 on the direct measurement of change in the capillary constant, 

 and the third involving, in addition, the knowledge of the 

 quantity of electricity required to decompose a gramme of 

 water; while they all involve in different ways a constant of 

 such large numerical magnitude as the ratio of the two elec- 

 trical units of quantity. 



9. The critical value 905 D in § 4 appears to have an im- 

 portant bearing on the much discussed question of contact 

 electrification. 



As was pointed out by Helmholtz, a discontinuous change 

 of potential in crossing a surface can only be produced by the 

 existence of an electrical double layer on that surface ; so long 

 as we look upon electrification or electric distribution as the 

 cause of electrical phenomena, this is the only explanation open 

 to us. 



It has been seen that this electrification represents a distri- 

 bution of purely surface-energy ; and if its properties are to 



