1916-17.] On some Nuclei of Cloudy Condensation. 
215 
XIV. — On some Nuclei of Cloudy Condensation. Part III. 
By Dr John Aitken, F.R.S. 
(MS. received March 12, 1917. Read March 19, 1917.) 
In communications to this Society in 1893* and in 1912,j* and in other 
papers, I have shown that some of the nuclei in the atmosphere have a 
power of condensing water vapour even when the air is unsaturated, owing 
to the material of which they are composed having an affinity for water. 
All kinds of nuclei have an affinity for water vapour, and by their surface 
action condense some water at all degrees of humidity, so causing increase 
in their size and hazing effect; but the particles having an affinity for 
water condense very much more and cause fogs in unsaturated air. In the 
present communication I intend to deal with the more common form of 
nuclei — that is, those which have little or no chemical affinity for water — 
and with those nuclei which require supersaturation to cause condensation ; 
the reason for this investigation being some statements which have 
appeared in scientific journals in this country and abroad regarding the 
nuclei counted by the condensation method. It has been definitely stated 
that the greater number of the nuclei counted are not dust particles but 
only ions, or aggregations of them. It therefore seemed desirable that 
further investigation should be made into the subject to see if there was 
anything to support this ionic theory of the condensation nuclei in the 
air, and at the same time to inquire into their origin. 
The best way of attacking this question seemed to be to make an 
investigation into the sizes of the nuclei in the atmosphere ; because if 
any of them are formed by the combination of ions, then we would expect 
to find in the air some nuclei of all sizes— from the size of ions which 
require a very high degree of supersaturation to make them centres of 
condensation, up to a size requiring only a very slight supersaturation. 
F or this purpose some new apparatus was necessary to enable us to sift 
and grade the particles into their different sizes, and to give some idea 
of their relative numbers. The method adopted of sorting out the 
different-sized nuclei was to make them centres of condensation at different 
degrees of supersaturation, all the larger ones being showered down with 
slight supersaturation, and the smaller ones with increasing degrees of it. 
* “ On some Observations made without a Dust-Counter,” Proc. Roy. Soc. Edin ., 1892-93. 
T “ The Sun as a Fog-Producer,” Proc. Roy. Soc. Edin ., vol. xxxii, part ii, No. 16. 
