536 Prof. J. J. Thomson on the Relation betiveen the 



that, if the transition from A to B, instead of being abrupt, 

 takes place through a thin layer (whose thickness need 

 only be commensurable with molecular distance) whose 

 properties are intermediate between A and B, the potential 

 energy due to surface-tension will be diminished. This dimi- 

 nution in the potential energy will produce a tendency for 

 such a transitional layer to be formed, even though the 

 formation of the layer is accompanied by a double layer 

 of electrification. The energy rendered available by a sen- 

 sible diminution in the surface-tension will be more than 

 sufficient to charge the double layer of electrification up to a 

 potential difference amounting to a large fraction of a volt. 

 For if the distance between the electrified layers is 10 " 8 

 centim., and their potential difference 1 volt, the energy per 

 unit area of the condenser formed by the double layer will be 

 equal to 10 16 /87r x 10~ 8 x 9 X 10 20 , or about 44 ergs. If the 

 surface-tension were the same as that of water, the energy due 

 to it would be 78 ergs per square centimetre, so that if any 

 sensible diminution in this took place, the energy rendered 

 available would suffice to charge up the layer to a potential 

 difference comparable with a volt. Even if the formation of 

 this layer required as a preliminary chemical decomposition to 

 liberate the charged atoms forming the layer, and this decom- 

 position required a supply of energy, there would still be 

 plenty available for this purpose. The thickness of the layer 

 is comparable with the range of molecular forces; thus the 

 quantity of matter in it, and therefore the amount of chemical 

 decomposition, will be exceedingly small, so small as to be 

 quite imperceptible by chemical means, but small as it is it 

 may, in consequence of the enormous electric charges carried 

 by the atoms, be sufficient to produce a finite potential differ- 

 ence between the coats of the double layer. 



The amount of energy which would be liberated by a di- 

 minution in the abruptness of transition, and the effects which 

 this might be expected to produce, are, I think, much greater 

 than is generally suspected. Thus we should, for instance, 

 expect that the abruptness of transition between, say, a metal 

 like mercury and a gas like oxygen, would be diminished if 

 between the mercury and the gas there was a film of the com- 

 pound of the two: the tendency to form this compound would 

 be very great, for if the film of oxide only diminished the 

 surface-tension by one per cent., enough energy would be 

 liberated to suffice to raise the temperature of a film of oxygen 

 10~ 8 centim. thick several hundred degrees Centigrade. With 

 this amount of energy available, it is difficult to avoid the con- 

 clusion that even the least oxidizable metals must, when ex- 

 posed to air, be coated with a thin film of oxide, and though 



