(oar) 
XII.—On Dust, Fogs, and Clouds. By Joun AITKEN. 
(Read, Part I., December 20, 1880; Part II., February 7, 1881.) 
Parr 1. 
Water is perhaps the most abundant and most universally distributed form 
of matter on the earth. It has to perform more varied functions and more 
important duties than any other kind of matter with which we are acquainted. 
From its close connection with all forms of life, it has been the subject of 
deepest interest in all ages. It is constantly changing from one of its states to 
another. At one time it is solid, now liquid, and then gaseous. These 
changes take place in regular succession, with every return of day and night, 
and every successive season ; and these changes are constantly repeating them- 
selves with every returning cycle. Of these changes, the one which perhaps 
has the greatest interest for us, and which has for long ages been the subject 
of special observation, is the change of water from its vaporous state, to its 
condensation into clouds, and descent as rain. Ever since man first “ observed 
the winds” and “regarded the clouds,” and discovered that “fair weather 
cometh out of the north,” this has been the subject of intensest human interest, 
and at present forms one of the most important parts of the science of 
meteorology, a science in which perhaps more observations have been made 
and recorded than in all the other sciences together. 
In the present paper I intend confining my remarks to this change 
of water from its gaseous or vaporous to its liquid state, with particular 
reference to that change when it takes place in the cloudy condensation 
of our atmosphere. Let us look briefly at the process as it goes on in 
nature. As the heat of the sun increases, and the temperature of the earth 
rises, more and more water becomes evaporated from its surface, and passes 
from its liquid form to its invisible gaseous condition; and so long as the 
temperature continues to increase, more and more vapour is added to the 
air. This increased amount of vapour in hot air compared to cold air is 
generally explained by saying that hot air dissolves more water than cold air. 
This, however, is not the case. Air has no solvent action whatever on water 
vapour. Water vapour rises into air to the same amount that it would do into 
a vacuum at the same temperature, only it rises into air more slowly than into 
a vacuum, and the amount of vapour which can remain in the air is independent 
VOL, XXX. PART I. 3 F 
