1911-12.] 
199 
The Sun as a Fog Producer. 
Silica Flask. 
I have here put these experiments on pure country air in their historical 
order, though from the physical point of view it may not be the best. 
In making these experiments with S0 2 and sunshine, glass flasks had 
been generally used A silica one had occasionally been experimented with, 
but little difference in its action from that of the glass flasks had been 
noticed ; and as good working results had always been obtained with glass, 
the use of the silica flask was not continued, since broken glass is more 
easily replaced than broken silica. Having found that neither purified air 
nor the pure air of Loch Awe gave any nuclei with S0 2 and sunshine — 
that the S0 2 in pure air was, so to speak, dead, as far as its power for 
harm with sunshine was concerned — one felt inclined to ask if all the 
conditions of the experiments were satisfactory ; did not this new knowledge 
about pure air call for a review of all the conditions of the tests ? And the 
glass flasks at once came under suspicion, as they stop certain of the sun’s 
rays. Experiments were therefore made with silica flasks. This was 
done on my return from Loch Awe, and only the impure air of that place, 
filtered through the caustic solution, was used. The result was now different. 
While the sun’s rays could form no nuclei in pure air after passing through 
glass, they proved to be powerful nucleus-producers after their passage 
through silica. 
The experiments with impure and pure air at first seemed to point to 
some other impurity in the air being necessary to make the S0 2 an active 
nucleus-producer with sunshine. Now we find, by using a flask more 
transparent than glass to the ultra-violet rays, that the sun can act on S0 2 
without, so far as we know, the presence of any other impurity. However, 
these experiments made with glass flasks are not without their teaching 
from a meteorological point of view. We see that the sun’s rays, if unchecked 
in any way, can produce nuclei out of S0 2 in pure air ; but the experiments 
made in glass show that if the rays of shorter wave-length are held back 
by glass, haze, or clouds, the sun will have no effect unless there are some 
other impurities in the air in addition to S0 2 In confirmation of this it 
may be stated that if there is only a good light and no direct sunshine 
the S0 2 and pure air in silica flasks give very little condensation, showing 
that clouds stop the rays that make S0 2 active in pure air, while good 
light without sunshine makes S0 2 active in impure air. We must always 
remember that when there is S0 2 in the atmosphere there are always plenty 
of the other impurities. 
The impurity in the atmosphere necessary to make S0 2 more active 
