Double-Layer of Solid and Liquid Bodies. 309 



escape from the metal. As to the external electrons or 

 negative ions (if any), they will be dragged into the double- 

 layer each time that they sufficiently approach it, and, 

 sticking in the positive side, may diminish the potential fall 

 at the surface. But neutral molecules may also stick near 

 or in the double-layer, and form additional double-layers of 

 the same or opposite sign. In those cases when the formation 

 of such extra double-layers is effected at the cost of the 

 potential energy of the primitive double-layer, the adsorbed 

 molecules must diminish the intrinsic potential of the metal 

 in proportion to its original value (but not, of course, beyond 

 zero). This appears to be in full agreement with the recent 

 experiments on contact electromotive forces *, and explains 

 their dependence on minute quantities of occluded (or rather 

 adsorbed) gases. 



It is clear that the contact electromotive forces, due to 

 the presence of surface electric double-layers, have nothing 

 to do with the thermo-electric forces generated by the 

 difference in the concentration and, consequently, the partial 

 pressure of the free electrons in adjoining layers. The 

 Peltier heat depends upon their kinetic energy and not upon 

 the energy of the rotating bound electrons forming the 

 atomic double-layer. 



In conclusion a few words must be said about the electri- 

 fication of insulators and electrolytes. 



Insulators are distinguished from conductors by the absence 

 of free electrons and not by the absence of intrinsic potentials, 

 which in this case may be positive as well as negative, de- 

 pending upon the structure of the molecules. A mere 

 contact between two dielectrics is insufficient for their 

 electrification, which will take place as soon as free electrons 

 are liberated near the contact surface. The old means of 

 doing this is friction. Indeed, the electrification of bodies 

 by friction was the first experimental fact discovered in the 

 phenomena of electricity. The exceedingly high potentials 

 reached in this way are simply explained by the fact that 

 the electric charges remain concentrated in those places only 

 where they have been formed (instead of being distributed 

 over the whole surface, as in the case of conductors), and 

 may be very large, thanks to the large capacity of the 

 condenser formed by the adjacent surfaces ; this capacity 

 enormously decreases when the bodies are torn apart, causing 

 a corresponding increase of the potential difference, which 

 may be initially equal to the contact electromotive force, 

 that is, to the difference of intrinsic potentials of the bodies. 



* See, for instance, Shaw, Phil. Mag. rol. xxv. pp. 255-256 (1913). 



