CH. VIIL] OF AN ELECTRON 83 



They can be got rid of most quickly, however, by intro- 

 ducing an electric field, that is to say by supplying 

 electrodes maintained at a few volts difference of 

 potential. They will then immediately make a 

 procession, as in electrolysis, only with much greater 

 speed, because their motion is much less resisted or 

 interfered with by chance collisions ; so they will 

 soon reach and cling to their respective electrodes, 

 and in that case again no true mist can form. But 

 it must be remembered that any of the numerous 

 causes of ionisation can produce some of them 

 again. 



While ions are present in considerable numbers 

 a thick mist will form whenever the space is 

 saturated with vapour, but it will be a mist 

 of different appearance from the slight rain-like 

 condensation which may be seen forming round the 

 few residual dust particles. The mist globules will 

 usually be of uniform size, and some estimate of that 

 size can be roughly attempted by the diffraction 

 colours which can be seen if a point of light is looked 

 at through the mist : not, however, a very easy plan 

 for making a trustworthy estimate.* 



Electrical nuclei can be produced in various 

 ways by anything, in fact, which dissociates the air 

 or which fills it with ions. Some are produced by 

 the splashing and spray of water ; some are given off 

 from flames, and from red-hot bodies ; they are 

 produced in considerable numbers when Kontgen 

 and when Becquerel rays travel through air ; they 

 can be given off by radio-active substances like 

 uranium; and they are easily emitted by a nega- 

 tively charged metallic surface exposed to ultra-violet 

 light. 



*See C. T. K. Wilson, Phil. Trans., 1897, A, vol. 189, p. 283. 



