Atom and the Charge of Electricity carried by it. 537 



this film is too thin to be detected, either by chemical or 

 optical means, it may yet, as we have seen, produce appre- 

 ciable electrical effects. The same effects, too, might be ex- 

 pected to take place between solids : thus, if copper and zinc 

 were brought into intimate contact, there would be a tendency 

 for a layer of brass to form between them, it does not seem 

 impossible that under favourable circumstances the zinc and 

 copper might be cemented together by this film of brass. 



This consideration, too, must have an important application 

 to the electrification due to the splashing of drops; this phe- 

 nomenon requires the existence of a double coating of elec- 

 tricity over the surface of the drop, to provide the atoms 

 which carry the charges on these coatings chemical decom- 

 position may have to take place. The energy for this may 

 come from the lowering of the surface-tension produced by 

 the formation of an intermediate layer, so that at the surface 

 of the drop chemical changes may take place, of which we 

 have at ordinary temperatures no experience from experi- 

 ments made with large masses of the substances. 



Contact-difference of Potential between a Metal and a 

 Coating of Oxide. 

 If we have a metal plate covered with a film of oxide or 

 other compound of the metal, the transition from the metal to 

 the air will be more gradual if the molecules of the com- 

 pound in the part of the film near the plate are arranged so 

 that the metal atoms are next the plate, and the oxygen atoms 

 on the far side. In the molecules of the oxide, the metal 

 atoms carry the charge of positive electricity, while the 

 oxygen atoms carry the negative charge : there will thus at 

 the surface of the plate be a double coating of electricity, the 

 positive side of this layer being next the plate, the potential, 

 therefore, will fall as we pass from the metal to the layer of 

 oxide, that is the oxide will be negative to the metal. This 

 agrees with the results obtained by those who have experi- 

 mented on contact-electricity, and it further appears, as a 

 possible interpretation of their experiments, that the potential 

 difference between a metal and its oxide may amount to a 

 considerable fraction of a volt. 



Communication of Electricity from the Electrified Atoms 

 of a Gas to a Conductor. 



This is only another aspect of the preceding question. 

 For suppose we have a number of electrified atoms of a 

 gas in contact with a metal, and that the metal is electro- 

 positive to the gas, the electricity can go from the gas to 

 the metal in the following ways : — (1) If the atoms of the 



