GASES BY THE ACTION OF EONTGEN RAYS AND OTHER AGENTS. 
431 
absolutely prove that such nuclei may not be formed by sunlight even in the lower 
layers of the atmosphere. For it is quite possible that the disappearance of the 
nuclei produced by weak ultra-violet light, when they are left to themselves, is 
entirely due to the fact that they very quickly reach the walls of the vessel by 
difiPusion on account of their small size. The time for whicli tliey persist would then 
entirely depend on the size of the vessel containing them. Now the explanation of 
the fact, that with weak radiation they never grow sufficiently to be caught 1)y slight 
expansions, may simply be that they reach the walls before any considerable growth 
has time to take place. In the atmosphere, according to this view they would persist 
for an almost indefinite time, and might finally become large enough to act like 
“dust” particles in helping condensation. 
In the preliminary note already puhlished"^ it was pointed out that in the upper 
regions of the atmosphere sunlight was likely to be rich in ultra-violet rays, and it 
was suggested that from their action on air and water vapour alone the small 
particles, to which the blue colour of the sky is due, might arise. TyndallI recog¬ 
nised the resemblance between the light of the sky and that scattered by the fogs 
which he obtained by the action of light on various vapours, pointing out that the 
light scattered by the fogs is polarised like that of the sky. He concluded that the 
blue colour of the sky was due to the presence of small particles like those produced 
in his tubes. The connection 1)etween the blue colour of the sky and tliat of the 
fogs produced from air and water vapour 1jy the action of ultra-violet liglit is possibly 
a still closer one, the small particles to which the colour is due having a similar 
origin in both cases. 
The cloud or nucleus-producing effect of ultra-violet rays has obviously Ijearings 
on other meteorological phenomena. The nuclei which enable clouds to form may in 
many cases arise from this source. The upper clouds especially may owe their 
formation in this way to the action of sunlight. It is possilde, too, that owing to the 
action of the ultra-violet rays, sunlight may sometimes cause clouds to persist in 
unsaturated air. 
§ 7. Nuclei produced by Metals, 
The presence of certain metals in the expansion apparatus was found to give rise to 
condensation nuclei. 
The apparatus used in most of the experiments was of the form shown in fig. 10a. 
(Only the cloud-vessel is shown, the rest of the expansion apparatus was the same as 
shown in fig. 1.) The cloud-vessel consisted of a portion of a wide test-tube. This 
was held in position by means of an iudiaruhber liand. The cloud-vessel was divided 
into two equal parts by a vertical partition, consisting in most cases of mica on one 
* ‘Camb. Phil. Soc. Proc.,’ vol. 9, p. 392, 1898. 
t Tyndall, ‘Roy. Soc. Proc.,’ vol. 17, p. 223. 
