NUCLEAR CONDENSATION OF CERTAIN ORGANIC VAPOURS. 
451 
expected that in the top partition the negative ions formed by the secondary rays 
are removed by recombination with the more numerous positive ions present there. 
Observation of the Drops. 
The expansion chamber was observed from E (fig. 4) in a direction making a small 
angle with the beam of light (from the incandescent gas mantle I), which was brought 
to a focus inside the chamber by means of the lens L. With the eye on a level with 
the diaphragm D, drops in both the toji and bottom partitions of the chamber could 
be observed at once on a background formed by black velvet. 
The switch in the primary of the induction coil, which worked the X-ray bulb at It, 
was placed close to the handle for “firing” the expansion apparatus, and so with a 
metronome beating half-seconds the Rontgen rays could be cut off at given intervals 
before the expansion was made. This metronome was also used for timing the rate of 
fall of the drops. 
Results with. Air and Water Vapour . 
The following results show that the expansion apparatus is capable of giving results 
suitable for absolute measurements. (The numbers represent expansions.) 
Air Initially Saturated with Water Vapour. Test-tube Expansion Chamber. 
No drops: D249, 1-253. Drops: D256, 1-329, D366 (many), R376 (fog). There¬ 
fore the least expansion for condensation on natural nuclei is R256 (l"247), and fog- 
point 1-376 (1-38). 
Rontgen Rays. Expansion Chamber with Aluminium Electrodes. 
1"232 no drops, l - 242 four drops seen, 1‘246 many drops, 1"246 many. Expansion 
for Rontgen ray ions 1‘246 (D247). 
The values in brackets are those obtained by C. T. R. Wilson. The agreement 
between the two sets of results is evidence that the expansion in the writer’s 
apparatus is sufficiently rapid to give good results. 
Selection of Liquids. 
The writer wished to calculate from the results obtained with the liquids he used 
the supersaturation, and the radius of the nuclei causing condensation, as Mr. Wilson 
has done for water. To do this it is necessary to know the saturated vapour 
pressure, tt, of the liquid at room temperatures and possibly 50° lower, and also its 
surface tension and density at this lower temperature. There are not very many 
3 M 2 
