222 Proceedings of the Royal Society of Edinburgh. [Sess. 
2 per cent. When the tests given in this table were made, no record 
was kept of the number of showers required to clear at 2 per cent, 
expansion. Its density was simply entered, and when there was no 
condensation this was noted. 
Consider the first test recorded, namely, outside air. The first condensa- 
tion at 2 per cent, was dense — that is, above the usual density given by air 
from that source. After all those nuclei were cleared away with 2 per 
cent, expansions, the expansion was increased to 4 per cent., when a suc- 
cession of expansions gave nine showers before condensation ceased. The 
expansion was then increased to 6 per cent., when three more showers 
were obtained ; and one shower was brought down with an 8 per cent, 
expansion. It will be noticed that in the second test the number of very 
small particles was much less, and that none were found quite so small 
as in the first sample. The third entry in the table is the result of a 
test of the air in a room in which gas was burning. In this case there 
were no nuclei requiring more than a 2 per cent, expansion to bring them 
down. It may be mentioned that before the gas was lit there were plenty 
of very small particles and the air was similar to the outside air at the 
time, but shortly after the gas was lit the small particles could not be 
detected. Two explanations may be offered of their disappearance. One 
is, that they may get entangled and brought down in the first dense 
condensations. The other is, that when particles become densely crowded 
the small ones tend to disappear, probably by aggregating to form larger 
particles. Tests were made to see if the former supposition was correct. 
A quantity of room air was diluted with filtered air so as to reduce the 
density of the first condensation and allow of the larger particles being 
cleared with fewer expansions, but this seldom showed the presence of 
very small particles. It seems possible that the great number of ions 
produced by the burning gas may attach themselves to the nuclei, and 
these in turn, owing to their electric charges, may attach themselves to 
each other. That is, the ions by charging the particles may cause them 
to aggregate to form large nuclei. 
Electricity and Nuclei. 
The nuclei produced by the electric discharge were now tested. The 
flask V between the filter and the test-flask had two wires led into it — one 
through the stopper, the other through a hole in the side — and cemented air- 
tight. Using a steel point and an electrophorus as a source of electricity, 
there were formed a few nuclei almost all requiring very high degrees of 
expansion to make them active (see test 4 in the table). The nuclei formed 
