1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
221 
tions made on the Rigi, when the mountain top was covered with 
cloud, points to the formation of even this dense form of condensa- 
tion in unsaturated air. It must he admitted that the Rigi 
observations of the humidity on this day are not very reliable, and 
the subject might be dismissed were it not that there are many 
observations made by other observers, on Ben Nevis and elsewhere, 
which point to the same conclusion. 
There is another point to which I can only call attention at 
present, as it has not yet been investigated. It has reference to the 
effect of the direction of the wind on the thickness of the air. It 
has been sometimes thought, while these tests were being made, 
that with certain directions of wind the air was unduly thick for 
the number of particles and the humidity. This would imply that 
the dust particles brought by certain directions of wind were larger 
or more hygroscopic than those brought from other directions. 
This seems possible, as it is the winds from inhabited districts that 
are suspected of being unduly thick. As these bring the sulphur 
products formed in burning coal, it seems possible they may be 
charged with vapour condensing particles. This point, however, is 
one that requires further investigation. 
Apparatus for Testing the Condensing Power of Dust. 
While on the subject of the condensing power of dust, or the 
affinity of atmospheric dust for vapour, I may here refer to some 
experiments made a year or two ago on this point. The principle 
I then tried to develop was to collect the atmospheric dust on glass 
plates, and test if water vapour condensed on the deposited dust 
in unsaturated air. The following was the plan adopted of carrying 
out this idea. First as to the method of collecting the dust. This 
was done by placing glass plates inside a thermometer screen, a 
I room, or wherever it was desired to get the dust to be tested. These 
plates were sometimes placed horizontally, and the dust allowed to 
I fall on them. Another plan of collecting the dust was to deposit 
it on the glass plates by the method used in my thermic filter. If 
it was desired to collect the dust inside a room by this method, the 
glass plate was placed vertically and in close contact with one of 
the panes of glass in the window. The plate was kept in its place 
by means of a little india-rubber solution. As this plate was colder 
