201 
1911 - 12 .] The Sun as a Fog Producer. 
active nucleus-producer, the test-flask showing a dense fog on expansion ; 
and at the same time there is also produced a little spontaneous condensa- 
tion. It may he here stated that this apparatus will be found to be a con- 
venient instrument for showing and measuring radio-activity. The action of 
radio-activity on the S0 2 is, like that of light, a cumulative one. If we only 
allow the radium, or whatever substance we are using, to act for a short 
time, the nuclei are small and do not give any condensation unless high 
expansions are used ; but if time be given for the radium to act, the 
nuclei can be grown to such a size that they cause condensation with the 
slightest supersaturation, showing that they have grown to the size of 
ordinary dust nuclei. One might imagine that in this case the radio- 
activity had converted the S0 2 into S0 3 by its action on the water vapour ; 
but this is not the whole case, because most of these nuclei have no affinity 
for water, and only condense when the air is slightly supersaturated. The 
small amount of spontaneous condensation nuclei may be due to the pro- 
duction of S0 3 . There is a curious and interesting point connected with 
these nuclei, similar to what was found with light. Working on the above 
method — that is, with the apparatus as in fig. 1 — in which filtered air is 
drawn into the apparatus, so that fresh supplies are used for each test, 
there is no difficulty in growing to the size of dust particles the nuclei pro- 
duced by radio-activity. But if we make the experiment with the tube 
apparatus, already referred to, in which the same air is used over and over 
again, it is found that after a time it is impossible to grow the particles to 
such a size that they will form nuclei with very slight expansion. Even 
after a long exposure to radium no condensation is produced without an 
expansion similar to that required for condensation on ions. It was noticed, 
however, that if the cloud particles produced under these conditions were 
evaporated by compression, they always left nuclei of dust-like size, as slight 
expansion afterwards gave nearly as dense a cloud as the one dissolved. 
The time of exposure to the action of radium is not given in these 
experiments, because the strength of the salt is not known. It may, how- 
ever, be stated that the radium salt used is capable of discharging an 
ordinary gold-leaf electroscope in a few seconds if held near it. With one 
minute’s exposure to this tube held close to the flask the S0 2 nuclei were still 
too small to form nuclei with very slight expansion, and some minutes’ action 
was required to make them large enough to give condensation with the 
slightest expansion. The rays from an X-ray tube were also found to act 
very powerfully on the S0 2 , causing very dense condensation on expansion. 
While treating of the action of radio-activity we may refer to the 
action of emanations from radio-active substances, though they ought 
