Atom and the Charge of Electricity carried by it. 529 



only to reach the first stage, that of the interchange of charges 

 between the atoms of the elements. If the process stops at 

 this stage, the electropositive substance will be charged with 

 positive electricity", the electronegative substance with nega- 

 tive electricity. If one of the substances, say the electro- 

 positive, is a solid or a liquid, while the other is a gas, then 

 at the surface of the solid or liquid there will be two layers, 

 one of positive electricity on the solid or liquid, the other of 

 negative in the gas. This double layer of electrification will 

 make the electric potential discontinuous at the surface 

 separating the two substances, the potential in the solid just 

 inside the surface of separation exceeding that in the gas just 

 outside the surface by a finite amount. The electrification 

 produced by the splashing of drops is evidence of the existence 

 of this double layer of electrification : when a drop of water 

 or of mercury falls through air and splashes on a plate, the 

 drop itself is positively electrified, while there is negative 

 electrification in the surrounding air. On the view just given, 

 this is an example of incomplete chemical combination: in the 

 case of the water, the electrical interchanges preliminary to 

 the formation of H 2 2 , the oxygen in the air getting the 

 negative charge, have taken place ; the second stage, however, 

 does not go on, or at any rate is not completed during the 

 falling of the drop. Thus the positively and negatively elec- 

 trified atoms are not bo and together so firmly as they are in 

 a chemical compound, and get separated in the great extension 

 of surface produced by the splashing. In the case of mercury 

 the first stage of oxidation, that is the assumption of the 

 negative charge by the oxygen and of the positive by the 

 mercury, occurs, while the second either does not occur or is 

 not completed during the fall of the drop. It would seem 

 from the phenomena of drops that examples of this incom- 

 plete chemical action are very frequent, for the splashing 

 of drops of the most widely different substances gives rise to 

 electrification. In fact it seems possible that the existence 

 of this double coat, and on our hypothesis of incomplete 

 chemical action, is almost universal at ordinary temperatures, 

 and that the electrification produced by the friction of hetero- 

 geneous substances is due to the partial rubbing off of this 

 coating from one or both of the substances. 



We shall now proceed to consider in the light of this 

 hypothesis as to chemical action several simple chemical and 

 electrical phenomena. We shall begin with the case of 

 oxidation. Let us suppose that we have a zinc rod immersed 

 in oxygen, and in order to take a case where the chances of 

 combination are greatest, let us suppose that the oxygen 

 molecules are dissociated by some external agent into atoms. 



