ALEXANDER MACFARLANE-RAINMAKING. 
73 
whole enclosed space, and in doing so it draws upon its internal heat. 
The loss may be so great as to chill the enclosed air below the tempera¬ 
ture at which the moisture saturates; if so, a cloud of particles of water 
appears. 
The conditions which determine whether the condensed vapor will take 
the form of fog, cloud, mist, or rain, were investigated first of all by 
Mr. John Aitken, of Scotland, in a brilliant series of experiments which 
commenced in 1880.* 
What is meant by fog, cloud, mist, rain ? The particles composing a 
fog are so fine that they scarcely fall through the air, a cloud is a little 
coarser in the grain, while a mist is still coarser in texture, and rain is 
any one of these while falling, whether it be a wetting mist or a drench¬ 
ing rain. Mr. Aitken showed that the dust particles in the air act as 
nuclei upon which the vapor may condense; if these are present in air 
cooled below the temperature corresponding to the moisture, condensa¬ 
tion takes place immediately. If the dust particles per cubic inch are 
very numerous, there are many centers of condensation and little water 
for each; hence fog. If the number is smaller, the nuclei being fewer 
get a larger share of water; hence clouds. If they are fewer still, mist 
ensues; and if they are very few, so much water condenses on each nu¬ 
cleus that a heavy drop is formed-; hence rain. If the air inside the re¬ 
ceiver has been thoroughly freed from dust, and then cooled by expansion 
below the dew point, a sudden shake in the operations will cause rain 
to form, the mechanical disturbance taking the place of the dust nucleus. 
If there were no dust in .the atmosphere, the rain would fall from a 
nearly cloudless sky, a phenomenon which has been observed at some 
places on the globe. 
The dust which is effective as a nucleus for the vapor in the atmosphere 
does not consist of the coarse motes which may be seen in the path of a 
sunbeam; it is microscopic, becoming visible only when its size is greatly 
increased by the load of water. Mr. Aitken said in his first paper: 
“In all probability the spray from the ocean, after it is dried and 
nothing but a fine saltdust left, is perhaps one of the most important 
sources of cloud-producing dust. It is well known that this form of dust is 
ever present in our atmosphere, and is constantly settling on every object, 
as evidenced by the yellow sodium flame seen when bodies are heated.” 
He further said: “ The composition of the dust will also be of great 
importance in determining its power as a cloud producer, as it is evident 
some kinds of dust will have a greater attraction for water vapor than 
others. Fine sodic chloride dust, for instance, we should expect would 
condense vapor before it was cooled to the saturated point, on account 
of the great attraction that salt has for water.” 
* Transactions of the Koyal Society of Edinburgh, Vol. XXX. 
