﻿414 Mr. H. Pealing on Condensation Nuclei 



explanation was that the effect reappeared every time the 

 apparatus was carefully cleaned. When, however, the glass- 

 wool (which had not been previously used) was cleaned, as 

 far as possible in the same way, it gave an effect which was 

 much less than it gave in the unwashed condition. Another 

 important flaw in the evidence was the fact that when once 

 the glass-wool had lost its property of producing nuclei, all 

 ^attempts to bring back its property of producing nuclei 

 failed. Obviously, as long as this was the case, the effects 

 in the two cases being so similar except in actual amount, 

 the evidence in favour of the explanation we gave could not 

 be regarded as conclusive. For this reason the present 

 investigation was carried out. Nearly all the experiments 

 were with glass-wool. 



Experiments with Glass- Wool. 

 First a word about experimental details. 



(a) The apparatus used was very similar to the Wilson's 

 apparatus used in the former experiments. 



(b) The apparatus was not cleaned with extreme care as 

 It was shown that the accidental impurities which glass gains 

 when left exposed to the air have no influence on the effect 

 investigated. The whole apparatus was thoroughly washed 

 with a strong solution of soap and water and then carefully 

 rinsed out with ordinary tap-water. Sometimes strong- 

 nitric acid was used before rinsing with the tap-water. 



(c) The size of the nuclei was estimated by observing the 

 pressure-fall in the Wilson's expansion apparatus necessary 

 to bring them down. As a rule a pressure-drop of 18*5 cm. 

 was necessary to catch the majority of nuclei, but very often 

 -a much lower pressure brought a large number of the nuclei 

 down. The number of nuclei was estimated in a rough 

 manner by finding out the density of the condensation cloud 

 produced when an expansion was made, the expansion 

 chamber of the apparatus being illuminated by the light 

 focussed from a Nernst lamp. 



(d) In order to eliminate causes other than the glass-wool 

 of producing the nuclei, the apparatus was left standing with 

 iodine in it until the effects produced on expanding the air 

 contained were the same as those obtained before the intro- 

 duction of the iodine. In other words, no experiments were 

 made with the glass-wool until the apparatus gave, when 

 saturated with iodine, the ordinary Wilson effects. 



{e) The effect produced by the glass-wool was observed by 

 passing iodine-laden air through a plug of it (about 25 cm. 

 long) direct into the apparatus. 



