260 Proceedings of Royal Society of Edinburgh. [sess. 
On the Particles in Fogs and Clouds. By John 
Aitken, Esq. 
(Read February 6, 1893.) 
{Abstract.) 
Cloud Particles. 
At the beginning of the paper some observations made on the 
water particles in clouds on the Eigi on the 21st of May last are 
described. Previous observations with the fog-particle counter 
had shown that there is a relation between the density of a cloud 
and the number of water particles observed. On the occasion above 
referred to the number was very much greater than corresponded 
with the density. It is pointed out that the number of dust par- 
ticles in the air which become centres of condensation depends on 
the rate at which the condensation is taking place, quick con- 
densation causing a large number of particles to become active, 
slow condensation causing a small number; and that after the 
condensation has ceased a process of differentiation takes place, 
the larger particles robbing the smaller ones of their water, owing 
to the vapour-pressure at the surface of drops of large curvature being 
less than at the surface of drops of smaller curvature. The particles 
in a cloud are by this process reduced in number, those remain- 
ing becoming larger and falling quicker, the cloud thus tending 
to become thinner by the reduction of the number of particles and 
by the falling of some of them. It is shown that the exceptional 
readings above referred to, obtained on the Eigi, were owing to the 
observations then made being taken in a new and rapidly-formed 
cloud, due to the strong wind blowing at the time causing a quick 
ascent and rapid cooling and condensation, the result being the 
formation of a large number of very small water particles. Though 
the number was very great, the particles were so small they were 
only just visible with great care with the magnifying power used 
in the instrument. Previous observations on cloud particles had 
been made in slowly-formed or in old clouds after the process of 
