1889-90.] 
Mr J. Aitken on Dust Particles. 
219 
between the transparency and the dust and humidity, because the 
air at the former place being polluted by local causes, it is probable 
there would be a want of uniformity in the air at different points 
near each other, and in the air from time to time. This was con- 
firmed by the observations. The numbers varied a good deal at short 
intervals. The effect of the wind in reducing the number of particles 
may be noticed also in the Kingairloch and Alford observations. 
There is a point of some interest brought out by a comparison of 
the Dumfries and Kingairloch observations to which it may be worth 
while directing attention. It is, that while the transparency of the 
air depends on the amount of dust, and on the humidity, or degree 
of saturation of the air, it would appear that the thickening effect 
of the humidity depends on the temperature, that is on the tension 
of the vapour. Dor instance, a humidity which gives a depression 
of 4° at a temperature of 60° has a much greater thickening effect 
than the same depression at 50°. This might have been anticipated, 
but it was only while the Dumfries tests were being made that it 
was thought that the air was clearer for the same humidity than it 
was at Kingairloch. An examination of the figures in the table 
supports this conclusion. Dor instance, take the following examples 
when the air had about the same amount of dust and about the 
same depression of the wet bulb. 
Date. 
Number. 
Humidity. 
Temperature. Transparency. 
5th July 
1500 
6 
64 
Thick. 
6 th July 
1000 
4 
61 
Thick. 
6 th Nov. 
1360 
4'6 
50 
Clear. 
29th Oct. 
1200 
4-6 
50-5 
Medium. 
Compare the above observations when the dust and the wet bulb 
depression were similar. In July, when the temperature was over 
60°, the air was thick, whereas in October and November, when the 
temperature was 10° lower, the air was clear or medium. The 
other figures mostly point in the same direction. The air observed 
on Ben Nevis would probably not have been nearly so clear if its 
temperature had not been low. 
This result I have said might have been anticipated, because 
at the higher temperature with the same wet bulb depression the 
vapour tension wfill be considerably greater than at the lower, and 
each particle will attach to itself a greater amount of water. These 
