226 Proceedings of the Royal Society of Edinburgh. [Sess. 
occasion the wind was generally E.N.E., and the lowest number observed 
was 500 per c.c. Though unfortunate in this respect, I was fortunate in 
another way. The E.N.E. wind brought to the window of the room where 
the tests were made the polluted air from two houses distant some few 
hundred yards. At first it looked as if the persistent E.N.E. wind was going 
to stop the work by bringing polluted air to the place of observation. 
However, on hunting about cross-wind- ways with the dust-counter I found 
a room where there was no local pollution and the numbers were low and 
constant. The test-flask was therefore taken to this window, filled with 
pure air, and returned to the operating-room and tested. The constant 
presence of polluted air at the window, instead of being a disadvantage, 
turned out to be an advantage, as it gave me an opportunity of com- 
paring newly polluted air with air free from pollution. It will be seen 
from Table II that not only did the locally polluted air contain a greater 
number of particles as counted by the dust-counter, and require a greater 
number of showers to clear at a 2 per cent, expansion, but it had more 
very small particles. While the locally polluted air had particles requiring 
8 per cent, quick expansion to make them active, the purer air had few 
requiring a 6 per cent, expansion. 
From these tests it would seem that time plays an important part in 
the disappearance of the very small particles, greater numbers being present 
in newly polluted air than in ordinary air. This disappearance of the small 
nuclei has been observed in laboratory tests. If we confine a quantity of 
air for a considerable time, the number of particles gets reduced, but the very 
small ones tend to disappear more quickly, and in many cases nothing but 
particles requiring only a 2 per cent, expansion remain. The very small 
particles seem either to get deposited on the sides of the vessel or become 
attached to other particles. It will be noticed that the number of particles 
counted by the dust-counter and entered in column 8 of Table II does 
not quite agree with the number of showers given by the new apparatus. 
One reason for this is that the tests were not made at the same time, one 
being made some time after the other, and so not made with the same air; 
and as the number varies quickly in polluted air, the differences are only 
what might be expected. 
One naturally asks what is the origin of these very small nuclei in the 
polluted air at Loch Awe. They came with the products of combustion, but 
to what are they due ? A partial answer to this question suggested itself 
while working at Loch Awe. On two occasions when the air in the room 
was fairly pure, and contained no particles requiring more than an expansion 
of 4 per cent, to make them active, a match was burned and the products 
