MR. T. H. LABY ON TEE SUPERS ATURATION AND 
466 
value of T could be found. The densities given*' were corrected for temperature by 
using the observed coefficients of expansion. In this equation R for oxygen would be 
2 - 6 x 1 0 6 , and for a gas of molecular weight M is 2‘6 x 10 ,J x 32/M. The molecular 
weight of acetic acid has been taken as 120, as found by Bineau and Ramsay and 
Young, t The values of c, the radius of a drop with charge e in equilibrium with its 
saturated vapour, in the preceding table are slightly different to those given previously, 
for c varies a little with the temperature, which in this table is t 2 , the temperature at 
which drops are formed in the expansion experiments. 
Discussion of Calculated and Observed Super saturations. 
In Calculating the supersaturation in the last table no account has been taken of 
the variation of surface tension with the thickness of the surface film. Reinold and 
Rucker’s observations show that the surface tension T of a water film begins to 
depend on the thickness when the film is about 10~ 5 cm. thick. For thicknesses less 
than this T diminishes, but passes through maxima when the film is U2xl0~ 6 and 
when 6xl0 -7 cm. (Johannot) thick. Lord Rayleigh, J however, found that one 
interpretation of his experiments led to the conclusion that an oil film only one 
molecule thick still had surface tension. The equation log (pf P) = O‘4lT/R0crc has 
been applied above to drops of about 7 x 10~ 8 cm. radius. It is to be expected that T 
for the surface films of such drops has a smaller value than for thick films. This 
would lead to a calculated value of p/ P, that is of S, which would lie too large—a 
difference that the last table exhibits, for in all but two cases (formic acid and methyl 
alcohol) the calculated value of S is larger than the observed. On this view we 
would conclude that the surface tensions of acetic, propionic, butyric, and iso-butyric 
acids do not vary with the thickness of the surface film, since the calculated value of 
S for these acids agrees with the observed. 
Condensation on Natural Nuclei. 
The least expansion producing condensation in the absence of Rontgen rays was 
determined with the mercury sealed test-tube expansion chamber of fig. 2. The 
following is an example taken at random of the observations obtained. 
Dust-Free Air Saturated with Ethyl Acetate. Natural Nuclei. 
Expansions ig/ig 
Without drops : L390, D506, 1*511, L5'24, R52G, 1531. 
With drops: 1-528, L534, 1 -536, U542, L545, L556, U587. 
Least expansion for condensation R528. 
* Van Nostrand’s ‘ Chem. Ann.’ 
f ‘Trans. Chem. Soc.,’ 49, p. 790 (1886) 
+ ‘ Phil. Mag./ October, 1899, p. 23 
