236 
Proceedings of the Poyal Society of Edinburgh. [Sess. 
A piece of the metal was polished and put in a glass cup suspended in the 
flask V, but it was quite inactive. It was thought this might be due to 
the rapid rate at which it got covered with oxide stopping the action 
before it could be tested. Arrangements were therefore made for polishing 
the metal while it was in the filtered air. A glass tube was passed through 
the stopper of the flask. At the lower end of this tube was an arrange- 
ment for holding a piece of the metal. Inside the tube was a rat- 
tail file fixed to a piece of rubber tube on the top of the glass one, by 
means of which the file could be made to polish the calcium without 
letting in dusty air. Further, as there was water in the flask, a deep cup 
had to be fixed under the metal to prevent the filings from falling into the 
water, so generating hydrogen and making nuclei by bubbling. The con- 
clusion come to was that if calcium is a nucleus-producer it is an extremely 
feeble one, as nothing but odd drops were obtained after filing. It must 
of course be remembered that the extent of surface cleaned was only 
2 or 3 sq. mm. 
Sodium and potassium were also tested, being placed in the glass cup 
in the flask V, but neither of these metals gave any nuclei. 
We see from these experiments on metals that phosphorus is not the 
only substance which gives off nuclei while cold. It is well known that 
phosphorus when oxidising gives off not only nuclei but also a great 
number of ions ; but whether the liberation of the nuclei by the\ metals was 
accompanied by ions was not determined, as the insulation of the electro- 
scope used was not good enough to show it. 
There is an interesting point connected with the behaviour of all very 
small nuclei, whether they be ions or very fine particles, and that is that 
after they have become centres of condensation they do not on drying 
return to their original condition. If we make a high enough expansion 
to cause condensation on the ions, or if we use a much higher expansion 
and get the fog-like condensation without ion nuclei, and if before the 
particles settle we return the pressure to its original condition, the 
cloud particles do not entirely evaporate, but leave larger nuclei behind. 
They will now be found to be active with a less degree of supersaturation. 
Some are active with a 2 per cent, expansion, and others require higher 
expansions, but none require very high. These particles soon disappear, 
probably owing to their electric charge. It is impossible to say that none 
of the nuclei return to their original state, but the density of the cloud 
which is given by a second expansion at 8 per cent, roughly corresponds 
to the density of the first cloud. The same thing happens when very 
small nuclei have once been made active with a high expansion : they after- 
