360 JOHN AITKEN ON 
of dust will have a greater attraction for water vapour than others, and that 
chloride of sodium dust would probably condense vapour and cause fogging in 
an atmosphere which was not saturated. 
There are evidently two ways in which dust may exert an attraction for 
water vapour, and determine its condensation while still unsaturated. The 
first is the attraction which the surface of some kinds of matter has for vapour, 
a power which they have of condensing a film of water on their surface. This 
power they possess at all degrees of saturation, but the amount they condense 
depends on the degree of saturation. Glass might be taken as an example of 
a substance whose surface has a strong affinity for water, a fact which dis- 
agreeably demonstrates itself in the conducting power of glass insulators of 
electrical apparatus in damp weather. The dust nuclei are so small that the 
condensing power of fine pores is not likely to have any influence. The other 
form of attraction which may exist between the dust and water vapour, is the 
chemical affinity which exists between the two. ‘This will evidently depend on 
the composition of the dust or nuclei. Asan example of this form of attraction, 
it will be sufficient here to mention the well-known affinity which chloride of 
sodium and other salts have for water, causing them to become wet when the 
air is moist. 
We shall presently see that besides these two ways in which nuclei may 
condense vapour in unsaturated air, there is another way in which the conden- 
sation may be produced in unsaturated as well as in saturated air without 
nuclei. This happens when there are gases or vapours present which have 
an affinity for each other, and the resulting compound is in a highly super- 
saturated condition. These new compounds under these conditions condense 
and form nuclei, which may be solid or liquid, and may or may not have 
affinity for water. 
Now it is evident that if there are any kinds of matter in the form of dust 
in the air which have an affinity for water vapour, they will determine 
condensation in unsaturated air. Some experiments were made to see to 
what extent cloudy condensation could be produced under these conditions. 
My first experiments were made by burning sulphur, and vapourising chloride 
of sodium. A small quantity of sulphur was lighted, and an open-mouthed 
receiver held over it for a few seconds, and then placed on the table. At first 
scarcely anything was visible, but after a time a decided haze made its 
appearance, and the density of this haze or fog was always in proportion to 
the moisture present in the air. The damper the air the thicker the fogging, 
and if the air was nearly saturated, the result was very remarkable. If the 
inside of the receiver was wetted so as to moisten the air, the sulphur products 
were a little more evident, and on placing the receiver on the table, a thin haze 
could be seen. After a time, however, this haze grew denser and denser, and 
