246 
Proceedings of Royal Society of Edinburgh. [sess. 
Conclusions . 
The following are some of the conclusions arrived at in this 
paper : — 
1st, The earth’s atmosphere is greatly polluted with dust pro- 
duced by human agency. 
2nd, This dust is carried to considerable elevations by the hot 
air rising over cities, by the hot and moist air rising from sun 
heated areas of the earth’s surface, and by winds driving the 
dusty air up the slopes of hills. 
3rd, The transparency of the air depends on the number of dust 
particles in it, and also on its humidity. The less the dust the more 
transparent is the air, and the dryer the air the more transparent it 
is. There is no evidence that humidity alone — that is, water in 
its gaseous condition, and apart from dust — has any effect on the 
transparency. 
4th, The dust particles in the atmosphere have vapour con- 
densed on them though the air itself may not be saturated. 
5th, The amount of vapour condensed on the dust in unsaturated 
air depends on the “ relative humidity,” and also on the “ absolute 
humidity ” of the air. The higher the humidity and the higher 
the vapour tension, the greater is the amount of moisture held by 
the dust particles when the air is not saturated. 
6th, Haze is generally produced by dust, and if the air be dry, 
the vapour has but little effect, and the density of the haze depends 
chiefly on the number of particles present. 
7th, Hone of the tests made of the Mediterranean sea air show 
it to be very free from dust. 
8th, The amount of dust in the atmosphere of pure country dis- 
tricts varies with the velocity and the direction of the wind. Tall 
of wind being accompanied by an increase in dust. Winds blowing 
from populous districts generally bring dusty air. 
9th, The observations are still too few to afford satisfactory 
evidence of the relation between the amount of dust in the atmo- 
sphere and climate. 
