209 
1911-12.] The Sun as a Fog Producer. 
from the sides of the flask. In cases of that kind it has been found useful 
to employ another lens to concentrate the light. This lens is held in the 
hand, and by its movements any cone of light it may form on the particles 
is caused to move, and we then know whether any suspected illumination 
is due to reflections from the flask or to particles in the air. 
A better form of test-flask has lately been used. It is simply a hollow 
prism-shaped bottle, one of the kind used for liquid prisms in spectrum 
observations. The flat vertical sides of the prism give fewer reflections, 
and the worked glass of the sides of the prism allow of clearer vision ; 
while the angle at which the two sides are set is an advantage for observing, 
as it directs the line of vision somewhat in the direction of the source of 
light and not at right angles to it. The movable condensing lens is also 
used with this prism-shaped flask, partly on account of the advantage of 
having a movable cone, and partly because it gives a brighter illumination. 
For observing, a double convex lens about 2 inches diameter and 2 inches 
focus, or a microscope with a 1-inch objective, will be found suitable. 
The second test to which the products from the gas flame were put 
was made in order to see if they produced any kind of nuclei which would 
cause condensation in supersaturated air after being exposed to sunshine. 
We have previously seen that the filtered products of burning sulphur and 
also the products from an ordinary clear fire gave dense condensation in 
supersaturated air after being sunned, though they gave none without 
the action of light. The products of this gas were therefore submitted 
to the same test. After leaving the cooling coil they passed through 
the filter into the silica flask, where they were sunned, being afterwards 
drawn into the test-flask, when they were expanded. It was found that 
the sunshine had produced no effect on them, no condensation taking 
place. Some peroxide of hydrogen was now added to the products, but 
even with it and sunshine supersaturation showed that no nuclei had been 
produced. 
These tests were made with the products of both luminous and non- 
luminous flames. They show that the products of purified gas do not 
form nuclei of spontaneous condensation, and that the filtered products do 
not form nuclei of any kind, behaving in both ways quite differently from 
the sulphur products. 
In experiments of this kind it is advisable always to have a check test 
on the results, in order to make sure that the apparatus is working correctly. 
For this purpose, after making each of the above tests on the gas products, 
a very little sulphur was burned and the products allowed to enter with 
the gas products into the apparatus and produce condensation. This was 
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