486 REPORT— 1894. 



electrostatic units. If r is the distance between the atoms in the molecule, 

 the force on unit charge at one of the atoms due to the other is e/r'-. If 

 we take r equal to 10"*, this force is equal to 10 '. To pull the atoms 

 apart would require a force comparable with this. As a force of 10* in 

 C.G.S. units corresponds to thirty million volts per centimetre, we see that 

 to separate the atoms in a molecule would require the application of an 

 electric force far transcending in intensity any hitherto applied to a gas. 

 A single molecule or a system of molecules free from each other's action 

 would, therefore, not be split up by the fields which are found to produce 

 discharge through gases. Such fields, however, might produce discharge 

 if there existed in the gas complex molecules from which the charged 

 atoms could be more easily detached than from isolated molecules. The 

 formation of these aggregates or large molecules with but loose connec- 

 tion between the atoms would on this view be an essential preliminary to 

 the passage of the dischai-ge. The presence of a third substance, such as 

 water, may facilitate the formation of these aggregates by supplying nuclei 

 round which they may condense. Indeed, a direct proof of the electrical 

 eflfect exerted by water on the surrounding gas is indicated by the electri- 

 fication produced when drops of water fall on a plate.' The most direct 

 explanation of this phenomenon is that when a drop of water is surrounded 

 by gas there is a finite difference between the potential of the gas and that 

 of the water ; in other words, there is electrical separation at the surface 

 of the drop resulting in the formation of a coating formed of two layers of 

 electricity close together, one of these layers being positive, the other 

 negative. If the gas surrounding the drop is oxygen, the inner layer is 

 positive, the outer negative ; while if the drop is surrounded by hydrogen, 

 the inner layer is negative, the outer one positive. Thus in a system con- 

 sisting of water and gas effects take place which result in an electiical 

 separation which becomes apparent when the drop receives such a com- 

 paratively trifiing disturbance as that produced by falling on a plate. 

 Water is, with the single exception of mercury, the liquid where the 

 electrification by drops attains the greatest dimensions. It is also the 

 substance that, as far as our present knowledge extends, produces the 

 greatest effect both on the electric discharge and on chemical combination. 

 As the electrification is carried by atoms, and as these can be separated 

 by the splashing of the drops on the plate, the electrification of drops 

 furnishes independent evidence of the ability of water to put the surround- 

 ing gas into a condition in which some of the atoms are but loosely attached 

 together. Although in this case the phenomenon is observed with liquid 

 drops of a finite size which we cannot suppose to have any pei/manent 

 existence in those cases where a trace of aqueous vapour produces such 

 an effect on the passage of electricity and on chemical combiiiation, yet, 

 if we realise this action of drops, we shall, I think, get a clue to the ex- 

 planation of the action of small quantities of aqueous vapour. For let us 

 suppose that we have a drop of water in air with its double layer of elec- 

 trification, and that this drop for some reason or another evaporates. 

 Blake's experiments show that the steam from this drop is not electrified, 

 so that if we proceed to the limit and suppose all the water to evaporate, 

 the charged atoms constituting the double layer will still be left, and we 

 shall have a number of oppositely charged atoms held together by very 

 loose ties. These would easily be split up themselves by the electric field, 



' Lenard, Wied. Ann., 46, 584 ; J. J. Thomson, Phil. Mag., 37, 341. 



