462 
MR. T. H. LABY ON THE SUPERSATURATION AND 
the effect of these radicals, though small, is proportional to the number of carbon atoms 
in them. Hydroxyl, H0-. This group is present in the alcohols which have the 
smallest values of S, and in the acids which have the largest values. It may be that 
when the -OH group is combined with carbonyl carbon in a compound that compound 
has a large S, while when it is combined with alkyl carbon the value of S is small. 
S must be known for more varied compounds than the above before this action of 
hydroxyl can be tested. 
Theory of Condensation on Ionic Nuclei. 
The theory of the condensation of drops has been given by several writers. 
Lord Kelvin’s^ well-known result showed that the surface tension makes the vapour 
pressure of a small drop greater than that of a large one. Professor J. J. Thomson,! 
in 1886, deduced an expression for the diminution in the vapour pressure of a drop 
when it is charged electrically. The combined effect on condensation of the surface 
tension of a drop and the charge it receives when it condenses on an ionic nucleus has 
been given by J. J. Thomson,! Bloch and Langevin,| and C. T. B. Wilson.§ 
Size o f Charged Drop in Equilibrium with Saturated Vapour. 
Professor Thomson finds that the vapour pressure of a drop as altered by its charge 
and surface tension is given by 
R01og e | = (—- 
P \ a 87rK« /cr 
( 6 ) 
when T is independent of a, where a is the radius, p the vapour pressure, 9 the 
temperature, e the charge, cr the density, and T the surface tension of the drop; R is 
the gas constant for unit mass of the vapour, P the pressure of the vapour in 
equilibrium with a very large drop; K is the specific inductive capacity of the 
dielectric surrounding the drop. 
When the drop is uncharged, i.e., e = 0, it will evaporate in the unsupersaturated 
vapour. When it is charged it increases in size until p/P = 1, i.e., log e (p/P) = 0, or, 
by (6), until 
a 
</ 
</ 
167tKT 
3-4 x 3-4 x 1(T 20 
16. IT. 1.76 
3-12 x 1 O' 8 cm. 
for water in C.G.S. units, taking e = 3'4 x 10 10 electrostatic units. 
* ‘ Proc. Roy. Soc. Edin.,’ 7, p. 63 (1870); also Helmholtz, ‘ Wied. Ann.,’ 27, 508 (1886). 
t ‘Applications of Dynamics to Physics and Chemistry,’ London, 1888; ‘Conduction of Electricity 
through Gases.’ 
| ‘Ann. Chem. et Phys.,’ [8], IV., p. 135, January, 1905. 
§ ‘ Smithsonian Report,’ p. 195 (1904). 
