H4 



VAPOR NUCLEI AND IONS. 



vessel by the sudden cooling incident upon exhaustion. This may be 



done by a straightforward approximation* with results shown in the 



following table, 45, where t t is the initial temperature of the saturated air 



within the fog chamber, t 2 the temperature after sudden exhaustion and 



before condensation, and / the temperature after the precipitation of the 



m grams of alcoholic fog per cubic centimeter. The drop in pressure is 



dp from p = 76 cm. at 20 C. The data of the last column will be presently 



explained. 



Table 45. Precipitation of alcohol vapor at different exhaustions (dp), super- 

 saturations (S), and radius (r), of nuclei. 



We may infer from the table that in a perfect apparatus and for true 

 pressure differencesf water fog particles would reach freezing (o C.) at 

 dp = 24 cm. and alcohol fog particles at ^ = 34-5 cm. Moreover, for the 

 same corona there must be on the average about 3.5 times more particles 

 in the alcoholic fog than in the water fog, which accounts for the opaque- 

 ness of the former. 



For the reasons adduced it is not worth while to express the results 

 otherwise than in round numbers, for the data involved are inevitably 

 crude. The assumption of the law of adiabatic cooling as far as - 36 C. 

 is questionable, in view of the admixture of saturated vapor; but as the 

 densities of vapor are for alcohol about 8 per cent that of air and for 

 water vapor about 7 per cent, this approximation in a rarefied atmos- 

 phere like the use of Boyle's law for a wet gas is probably admissible. 

 It is different, however, with the latent heat of the vapor, which is 

 required at the low temperatures, but is known (as a rule) only at 

 temperatures near the boiling-point. From this and similar points of 



*C. T. R. Wilson: Phil. Trans., London, vol. 189, 1897, p. 300. 



t dp computed as p-p 2 in the way shown in Chapter II, and not the apparent value 

 observed at the fog chamber. 



