Atom and the Charge of Electricity carried by it. 535 



the sign of the contact potential difference between two sub- 

 stances does take place when the circumstances are altered in 

 such a way that we might expect great changes in the relative 

 importance of these effects to occur. Thus there are cases 

 where, when the substances are cold, when we should expect 

 the chemical action to be sluggish, the electropositive sub- 

 stance is at a higher potential than the electronegative one, 

 while when the temperature is that of red heat or higher, 

 when chemical action is presumably more vigorous, the sign 

 of the potential difference is reversed. Thus, for example, 

 when a drop breaks off from a column of mercury and falls 

 through oxygen, the mercury is positively, the oxygen nega- 

 tively electrified : again, when ultra-violet light falls upon 

 a clean metal plate, the metal is positively, the surround- 

 ing air negatively electrified. In this case the. sign of 

 the potential difference indicates that the effect due to the 

 interchange of charges overpowers that due to the chemical 

 combination. If, however, we have a piece of metal above a 

 red heat surrounded by air, the air will acquire a positive 

 charge, the metal a negative one ; so that in this case the effect 

 of chemical combination overpowers any effect that may be 

 due to the interchange of atoms. 



The contact of the zinc with the solution in the preceding 

 example thus gives rise to the formation of two layers of 

 atoms, one layer being positively, the other layer negatively 

 electrified. When the solution is very weak, this double layer 

 which is interposed between the zinc and the solution consists 

 of a layer of partly dissociated zinc molecules, the positive 

 atoms being next the solution, the negative atoms next the 

 zinc plate. If the solution is strong, the transition between 

 the zinc and the solution is made by a layer of partly dissociated 

 molecules of the salt, the positive layer, consisting of zinc 

 atoms, being next the zinc, the negative layer, consisting of 

 atoms of the electronegative constituent of the salt, being 

 next the solution. In the case of the contact of a metal and 

 the electrolyte, the work required to produce the double 

 electrical layer which gives rise to the potential difference is 

 in the preceding investigation supposed to be done by the 

 osmotic pressure. There is, however, another source of energy 

 quite adequate to produce potential differences at the surfaces 

 of contact of different substances, and which is available in 

 the case of solids as well as liquids. This source is surface- 

 tension. Every unit of area of the surface of separation of 

 two substances, A and B, contributes an amount of potential 

 energy equal to the surface-tension between A and B. Now 

 Lord Rayleigh has shown (Phil. Mag. October 1883, p. 315) 



