416 MR. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
35-37.) They do, however, last longer than the nuclei produced by X-rays, When 
the nuclei are large enough to be caught by a very slight expansion they last for 
many minutes at least (Expt. 2). As will be seen later, those produced by very 
strong radiation last for many hours. The shorter life of the smaller nuclei is 
probably due mainly to their more i-apid rate of diffusion. 
4’he limit to the size attained by tbe nuclei for a given strength of radiation when 
tlie time of exposure is indefinitely prolonged is, perhaps, also to be explained by 
diffusion. For, if the radiation be weak, the nuclei may reach the walls by diffusion 
l)efore any considerable growth has time to take place; whereas, - with stronger 
radiation, not only will the drops grow more in a given time, but the slower rate of 
diffusion resulting from their increased size must increase the time for which they 
remain exposed. It is not surprising, according to this view, that a comparatively 
small increase in the intensity of the radiation may result in a very great diminution 
in the least expansion recpiired to catch the nuclei. 
Fig. 3. 
• B 
Experiments were now carried out with the object of deciding whether the nuclei 
are produced throughout the volume of the moist air, or, only at the surface of the 
c[uartz, as Lenaed and Wolfe supposed {Joe. cit.). For this purpose an expansion 
apparatus was made of the form shown in fig. 2. The ultra-violet light entered from 
])elow through a cpiartz plate, which was covei'ed with watei- to a depth of about 
5 millims. Tlie (pmrtz plate was held up against the ground edge of the cloud- 
chamber 1)y means of two elastic bands, an indiarubber washer being used as before 
to make an air-tigbt joint. The a])paratus was wrapped in tinfoil, provided with the 
]iecessary apertures. 
