478 Proceedings of the .Royal Society of Edinburgh. [Sess. 
close to B, an increase in the number of drops will take place, it may 
amount to a slight shower or a dense shower or a fog, according to the 
strength of the radioactivity. These new drops are condensed on the ions 
formed by the radioactive body. Wilson finds that the nuclei always 
present in moist air, and these ions formed by the radioactive substance, 
require the same expansion to cause them to become nuclei. My 
experience, however, does not fully confirm this. This difference in our 
conclusions may be due to the difference in our apparatus. There, how- 
ever, may be a different explanation. It has been observed that if the 
apparatus here described be left for some time, that on making the first 
expansion after the rest that the number of drops on the natural nuclei is 
greater than that given before stopping work, and greater than that 
obtained on subsequent expansions ; but as the degree of expansion with 
this apparatus may alter with time, the conclusion is not certain. In 
Wilson’s experiments it is probable there were longer intervals between 
the expansions, and this would bring the expansion required by the 
natural nuclei near to that required by the ions. If this explanation be 
correct, then it looks as if these natural nuclei became larger or at least 
better centres of condensation with time. 
With the apparatus shown in fig. 1 it is found that if the compression 
is reduced till there is hardly a drop visible, that the instrument is quite 
as sensitive to radioactive substances as with higher compressions. Of 
course, there are not so many drops formed, because with higher expansions 
the drops produced on the natural nuclei are added to those produced by 
the radioactivity. But, making allowance for these natural nuclei, there 
does not seem to be more due to the ions than when the lower compression 
was given. The question might be asked here, Does the radioactivity act 
on the natural nuclei and increase their condensing power and cause them 
to become active with lower supersaturations ? It is evident that the whole 
of the nuclei due to radioactivity are not produced in this way, because, 
with comparatively feeble radiation, the number of drops far exceeds the 
densest shower given by the natural nuclei. 
The action of the radioactivity is to produce ions in the air in the 
chamber B, and these ions become centres of condensation on supersatura- 
tion. The ions produced by the radiations have a very short life. If we 
remove the radioactive substance before the expansion is made the 
density of the condensation is greatly reduced. If we allow two seconds 
to pass before the expansion is made, only a slight effect remains. After 
five seconds all the ions are gone, having combined with each other. The 
continued action of the radioactive substance does not make the ions 
