1892-93.] Mr J. Aitken on Particles in Fogs and Clouds. 261 
differentiation had been in play for some time, and after the drops 
had been reduced in number and increased in size. 
Fog Particles. 
Fog particles, when formed in pure air, conduct themselves like 
cloud particles, and they have much the same appearance and size 
when seen on the micrometer of the fog-particle counter. Differen- 
tiation takes place in the particles of fogs also, and after the 
fog has been formed some time the particles are large and fall 
rapidly. Some fogs seem to clear away in this manner by raining 
themselves out of existence. But in town fogs the conditions are 
much more complicated, owing to the impurities in the air caused by 
the formation, during the combustion of coal and other causes, 
of a great number of nuclei which have an affinity for water vapour. 
This affinity of the nucleus for vapour not only determines condensa- 
tion and thickening of the atmosphere before it is saturated, but after 
condensation has taken place this affinity resists the differentiating 
process. The affinity of the nucleus for vapour resists the tendency 
which the smaller particles have to evaporate, and the differen- 
tiating process is stopped when the particles are reduced to a certain 
size. The number of particles in a town fog thus tends to be great, 
and to remain great, while in a country fog they tend to be few, 
and to fall. There are thus persisting fog pai'ticles and particles 
which tend to vanish. The one clings the stronger to water the 
smaller it is, the other parts with it the freer the smaller it is. The 
affinity of the nucleus thus not only increases the density of the 
fog, but it prevents the natural decay. 
These points were illustrated by filling two large glass receivers 
with air containing different kinds of nuclei. One receiver was filled 
with the products of combustion from a paraffin-lamp, the other 
with the products from a gas-flame, in which was burned a very 
little sulphur. The two receivers were connected by means of tubes, 
so as to be under the same conditions as regards cooling when 
the air in them was expanded. 'When the pressure was slowly 
lowered, the chilling produced a condensation in both receivers, but 
the one containing the sulphur in the products was much the denser. 
To show that the air in the receiver containing the lamp products 
