210 
Proceedings of Royal Society of Edinburgh . [sess. 
only an increase in tlie number of particles in the air, but there is 
also an increase in the depth of the polluted stratum of air, over the 
city. The Eiffel Tower tests showed that the polluted air of Paris 
rose to a height of more than 1000 feet. Probably on the day the 
tests were made it may have extended to many times that height, as 
extremely polluted air came to the top of the tower from time to 
time. The larger the city the greater will be the mass of heated 
air rising over it, and the greater its ascending power, so that the 
depth of polluted air leaving a city must be very great, while that 
leaving a small town will be much less. So that when we add to 
our knowledge of the number of particles, the greater depth of the 
polluted stratum of air over cities, we are better able to understand 
why the air of cities is not more impure than the tests show it to be. 
When the Eiffel Tower tests were made the air was fairly dry, 
which favoured the ascension of the heated air. When, however, 
the air is damp, it seems probable that the products of combustion 
will become loaded with condensed moisture, and this will counteract 
the ascending tendency of the heated air, and the thickened air 
during this weather will probably keep lower down and give rise to 
the dull thick town atmosphere of damp weather, and to town fogs 
when the -wind falls, especially if the condensation is intensified by 
the cold produced by radiation. 
It may here be asked, Is 200 particles per c.c. the lower limit 
of the purity of our atmosphere, or is there such a thing as air 
absolutely free from dust % The air at extreme elevations never has, 
and probably never will be tested; but nature has provided a test 
which shows that even at very great heights dust is present. Where- 
ever a cloud forms we know there is dust, and as clouds form at 
great elevations we may conclude that dust is present at even these 
great heights. It has been shown in a previous paper that conden- 
sation may take place in dust-free air ; but while this is so, yet the 
conditions under which this is possible are such as never happen 
in nature, because before it can take place the air requires to be 
greatly supersaturated, and be put into rapid eddying movements, or 
receive some violent shock, a condition of things never likely to 
happen in the upper strata of the atmosphere. And further, if 
condensation did take place, it would not take the cloudy form but 
the rainy one ; that is, the centres of condensation would be few 
