452 MR. C. T. R. WILSON OX THE CONDEXSATIOX NUCLEI PRODUCED IN 
molecular dimensions. This again is an agreement with experiments on the velocity 
of the ions."^ 
It is, perhaps, of some interest to calculate what charge would be required to keep 
a drop of the above-mentioned size (radius = 8'6 X 10"® centim.) from evaporating. 
Making use of the results given by Professor Thomson (‘ Applications of Dynamics,’ 
pp. 163, 165), we have, when the charge on the drop just balances the effect of 
surface tension so that there is no longer any tendency to evaporate in vapour 
saturated with respect to a flat surface, 
e® = 167rTcC, 
where e is the charge on the drop, T is the surface tension, and a is the radius. 
This gives us in the present case 
e = 1-5 X 10"° electrostatic unit, 
which agrees sufficiently nearly with what we have reason to suppose the order of 
magnitude of ionic charges to be. We must not forget, however, the assumptions 
made in obtaining the above-mentioned estimate of the size of the nuclei {vide ‘ Phil. 
Trans.,’ loc. cit., p. 305). 
Townsend has showiif that freshly prepared gases are often electrified, and that 
the charge is carried ])y nuclei on which, even if the gas be not saturated with 
aqueous vapour, water condenses to form visible drops. He has shown, moreover, 
that the charge carried by each of these nuclei (in oxygen) amounts to about 
3 X 10"^° electrostatic unit, and is presumably the charge carried by one ion. The 
experiments of H. A. Wilson^ furnish strong evidence that the growth of the 
drops in the fogs studied by Townsend is not, however, a direct consequence of the 
charge which they carry, but is due to the presence of some substance in solution in 
the drops. 
Many of the cases of condensation (apparently with only slight or without any super¬ 
saturation) produced by chemical action, which were studied by II. v. Helmholtz 
and Richarz,§ and which were attributed by them to the influence of free ions, are pro¬ 
bably also mainly the result of the formation of some substance in solution in incipient 
drops (of which the original nuclei may be free ions). Professor J. J. Thomson] | has 
suggested that the great influence which the presence of moisture has in facilitating 
chemical reactions between gases may be due to the presence of minute drops, at the 
surface of which (or throughout the volume) the combination is able to take place. 
For example, dry NH 3 and HCl do not combine, but if water vapour be present, 
* Rotherford, ‘ Camb. Phil. Soc. Proc.,’ vol. 9, p. 415, 1898. 
t Townsend, ‘ Camb. Phil. Soc. Proc.,’ 9, pp. 244 and 345, 1897. 
X H. A. Wilson, ‘Phil. Mag.,’ vol. 45, p. 454, 1898. 
§ Helmholtz and Richarz, ‘ Wied. Ann.,’ vol. 40, p. 161, 1890. 
II J. J. Thomson, ‘Phil. Mag.,’ vol. 36, p. 313, 1893. 
