1911-12.] 
195 
The Sun as a Fog Producer. 
drop being seen on expansion ; and accordingly it may be kept a long time 
without producing any nuclei if it is kept under certain conditions. We 
shall therefore first consider some of the outside influences which make the 
S0 2 a nucleus-producer, as we have to guard against them when considering 
the effects of other gases on it. The first of these influences we will 
consider is 
Light. 
The S0 2 only remains free from nuclei if we keep it in the dark. If 
the flask S is exposed to light, especially sunshine, then a change is effected 
which converts the acid into an active nucleus-producer. If some of the gas 
be sunned in S and drawn into the test-flask and expanded, a very dense 
condensation takes place. Some of these sun-formed nuclei are of the kind 
that form spontaneous condensation, showing not only that the sunshine has 
formed nuclei, but that some of them have an affinity for water. It has 
also been found that the sun makes more nuclei of spontaneous condensa- 
tion when the products of burning sulphur are used in place of the solution. 
The products of the burnt sulphur were in this test drawn into the apparatus 
through the filter. Further, it has been found that this action of light is 
a cumulative one , the particles growing in size under the continued influence 
of light. If only a short exposure to light be given, then the particles are 
so small that they require a considerable supersaturation to make them 
nuclei, and it is necessary that the action of the light should be continued 
a minute or two, according to its strength, to increase the growth of the 
particles to such an extent that they become active nuclei with only slight 
supersaturation. This change of the S0 2 is also produced by the light 
of burning magnesium, but only very slightly by light from an incandescent 
gas mantle. 
These experiments on the action of light on S0 2 vapour remind one of 
Prof. Tyndall’s work on the decomposition of some vapours by the electric 
light. This action on S0 2 is, however, evidently something different from 
decomposition. There is the growth of the nuclei to be explained ; and, 
further, if the S0 2 were decomposed the sulphur nucleus would have no 
affinity for water. It would rather seem that the effect is due to some 
combination of the S0 2 molecules taking place with other gases present, 
which will be referred to later on. 
The conditions of the experiments on sunlight and S0 2 were now 
changed. In place of the apparatus shown in fig. 1, the apparatus designed 
for showing radio-activity, described in a paper read before this Society,* 
* Proc. Boy. Soc. Edin vol. xxix., Part V., No. 30. 
