240 Proceedings of the ldoyal Society of Edinburgh. [Sess. 
produced plenty of ions, as was evidenced by the density of the showers 
produced by an expansion of 26 per cent. The nuclei produced by these 
cloud particles were then cleared away, and the air left free of nuclei of 
any size larger than ions. It was found that the radium might be left for 
an hour, or a day, without the ions combining to produce particles requiring 
an expansion less than that required for ions. The ions evidently did not 
grow in damp air, so the conditions were altered. The flask V was put in 
its place, and the small bottle of the radium with the cork removed was put 
in it and the dusty air cleared out. After hours of exposure in the dry air 
to the action of the a , /3, and y rays, there was no condensation till the 
expansion was high enough to act on the ions. A bottle with hydroxide 
of thorium with the cork out was put in V in place of the radium. The 
emanations from this body are very powerful ionisers, yet it gave no nuclei 
larger than ions. So that neither in dry nor in moist air do the ions show 
any signs of combining to produce large particles. I wish now to refer to 
an experiment on this subject described in Part I.* In that experiment 
pure hydrogen was burned in Altered air. Now, the products from that flame 
gave no nuclei even with fairly high expansions, and yet there must have 
been large quantities of ions in it. It is true the time for combination was 
short ; but as the ions would be very numerous, one would have expected 
some combinations to have taken place. » 
It is not contended that because the ions under the above conditions 
did not combine to form larger nuclei, therefore they never do so. But 
it fairly rests with those who uphold that theory to show under what 
conditions they do combine. Further, it is not contended that ions may 
not under certain conditions cause the formation of large nuclei. My own 
experiments described in a previous paper j* show that they do. If, for 
instance, there be sulphurous acid and other impurities in the air, then 
enormous numbers of large particles are produced by ions. These, however, 
are not aggregations of ions, but are due to the ions bringing about a 
chemical action which results in the formation of large particles. 
From these artificial conditions let us now turn to nature. An examina- 
tion of the figures in Tables I and II shows that there are no very small 
particles in the atmosphere. Now, if the nuclei of cloudy condensation were 
really made up of aggregations of ions, there ought to be particles of all 
sizes in the air, from ions upwards. This, however, is not the case ; seldom 
are there any particles requiring more than a 6 per cent, expansion to 
* “ On some Nuclei of Cloudy Condensation,” Trctns. Roy. Soc. Edin ., vol. xxxix, part i, 
No. 3. 
j “ The Sun as a Fog-Producer,” Proc. Roy. Soc. Edin., vol. xxxii, part ii, No. 16. 
