482 Proceedings of the Royal Society of Edinburgh. [Sess. 
nected up, the first thing to be done is to get rid of the dust particles in 
B. This is done by pressing the lever G and releasing it, when a fog will 
be seen slowly settling in B : time is given for it to settle, then another 
compression and expansion is made, when more cloud particles appear. 
This process of compression and expansion is continued till all condensation 
ceases in B with slight compression. The stop J on the compressor is 
now fixed at any point on K and the lever G pressed down till L engages 
with the stop J. The compression is left on for a time, and then the trigger 
K is pulled back, the lever L released, and the air in B expanded. While 
the air is expanding the eye of the observer should be watching the 
micrometer through the lens. If no drops appear, then the expansion has 
not been enough to produce condensation in dustless air : the stop J must 
therefore be pushed further down and another trial made, and so on till 
some drops appear on the micrometer. Suppose, on the other hand, that 
on first trial there should appear a number of drops, then the compression 
must be reduced. It will be found that the best degree of compression for 
these experiments is that which gives just an odd drop or two visible over 
the whole field. In that condition, though the expansion is not enough to 
bring down all of what we have called the natural nuclei in the air, yet it 
seems to be enough to bring down all the ions formed by the radioactivity. 
It will, of course, only bring down the negative ions, as Wilson has shown 
that while negatively charged ions become active nuclei when the expan- 
sion is represented by V 2 /V 1 = 1*250, while the expansion must be as great 
as V 2 /Y 1 = l*31 to bring down the positive ions. If the expansion is in- 
