CH. VIIL] OF AN ELECTRON 81 



that a single grain of lycopodium powder contains 

 about a trillion atoms, and a dust particle big enough 

 to condense vapour need not consist of more than a 

 billion, or perhaps not more than a million, atoms, 

 and need by no means be big enough to be visible 

 even in a microscope. It is, however, material enough 

 to be stopped by a properly packed cotton-wool filter. 



J. J. Thomson and Electrical Nuclei. 



In 1888 it was shown by J. J. Thomson, in his 

 book Applications of Dynamics to Physics and 

 Chemistry, p. 164, that electrification of a body 

 would partially neutralise the effect of curvature, and 

 so assist the condensation of vapour on a convex 

 surface. 



Consider a drop of liquid, or a soap bubble ; the 

 effect of the curvature of the surface is to give a 

 radial component of surface tension inwards, causing 

 an increased pressure internally. The effect of 

 electrification is just the opposite : it causes a direct 

 pressure outwards, which goes by the name of electric 

 tension. 



The way these depend on size is as follows : 



The radial-pressure component of the surface- 

 tension T is 



inwards. 



r 



The electric tension is 



outwards. 



They are differently affected, therefore, by the size 

 of the globule ; hence at some size or other they must 

 balance, and such an electrified convex surface will 



L.E. F 



