82 DETERMINATION OF THE MASS [CH. vni. 



behave as if it were unelectrified but flat. Accord- 

 ingly vapour which would refuse to condense on an 

 unelectrified convex surface, until far below the dew 

 point, will begin to condense on it, if sufficiently 

 electrified, the instant the dew point is reached. 



The critical size at which the ionic charge enables 

 a sphere of water to act as regards condensation as if 

 it were flat, can be reckoned by equating the pressure 

 to the tension, thus : 



r Sir KT* 

 e 2 10- 21 1 





whence r=10~ 8 approximately, or is of atomic 

 magnitude. 



Hence ions may be expected to condense vapour 

 at the ordinary dew-point ; and anything bigger 

 which possesses the same charge can condense it 

 still more easily. 



Accordingly an electric charge assists vapour to 

 condense ; and a sufficient electric charge might 

 cause it to condense on quite a small body as 

 small even as an atom. Hence in the presence of 

 ions, dust particles are not necessary for conden- 

 sation. - Vapour may condense on these electrical 

 nuclei without the need for solids of finite curva- 

 ture. The electrical nuclei cannot be completely 

 filtered out by cotton wool : they will exist or 

 can be produced in dust-free air. No doubt if 

 they are passed through a great amount of metal 

 gauze they may be diminished in number, but 

 they are not easily got rid of except by their 

 own diffusion, which does ultimately enable them 

 to pair off or to migrate to the sides of the vessel. 



