243 
1916-17.] On some Nuclei of Cloudy Condensation. 
the speck of cotton gave many, it is that while the burning hydrogen 
added nothing to the air that was not there before, it only increased the 
humidity and the number of ions ; while the cotton introduced new 
elements, and the possibility of chemical action taking place in the gases. 
Or the particles might be due to impurities on the cotton, forming nuclei as 
in the case of the heated iron wire. 
There is a difficulty which I have always felt with regard to the calcula- 
tion of the size of nuclei. The radius has been calculated from the super- 
saturation required to make them centres of condensation — in other words, 
the higher vapour pressure due to their convex curvature over that at a 
flat surface. But when one comes to ask, What are we to measure to get 
the size of the nucleus — suppose it is a small solid particle : is it the radius 
of the solid ? This seems doubtful. It is generally admitted that on the 
surface of all solids there is condensed a film of any gas or vapour that 
may be present. Now, though this film may be extremely thin, yet two 
thicknesses of an extremely thin film may add very materially to the 
diameter of the very small particles. Whatever its action may be, it 
introduces a disturbing element into the calculation of their size. For 
instance, the wonderful nucleus-producing power of phosphorus seems to 
depend greatly on its power of condensing water vapour. This condensing 
power can easily be seen when it is oxidising in the presence of wet 
surfaces, as it forms a heavy fog ; but it is only when the particles are 
closely packed and probably grown to some size that this fog takes place. 
In the tests with this substance, when the air was quickly drawn away 
from it there was no such action, yet its affinity for water enabled the 
very small particles to become active with but slight expansion ; but if a 
short time was given for the particles to increase in numbers and to grow, 
they gave a fog without expansion which became very dense with the 
slightest expansion. The particles in this case must have been extremely 
minute, as the piece of phosphorus from which the thousands of nuclei came 
was no larger than a pin point ; and their activity would seem to depend on 
their affinity for water vapour making them very much larger. The above 
speculations are advanced with considerable hesitation, as I have not the 
knowledge necessary for forming an opinion on the points, and I leave 
them as a suggestion. 
In this investigation it has been shown that there is no proof that 
ions alone ever form nuclei large enough to cause condensation with but 
little supersaturation, and there is no evidence that large particles are ever 
formed by the ions in the atmosphere, as in pure air there are no nuclei 
smaller than can be brought down with a slight expansion. The small 
