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1911-12.] The Sun as a Fog Producer. 
condition as when it entered, but is affected by the condition of the air 
which previously passed through. If the previous air was purer, the 
sample will be too pure; if it was more impure, the sample will be too 
impure. And, further, it is doubtful whether small quantities of sulphurous 
acid are not entirely destroyed by the filter. So these methods of testing 
do not seem likely to give a satisfactory answer to our question. 
The other method of investigating is to test the air for nuclei of 
spontaneous condensation. First note the number of particles the air 
gives when drawn into the test-flask and saturated with water, then add 
to the air some hydrogen peroxide and expose to light, and see if there is 
any change in the number of particles formed by spontaneous condensation. 
If so, then it may be concluded that there is in the air some gas capable 
of producing fog under the influence of sunlight. A third method of 
testing is to use a dust-counter and first find the number of particles in 
the air, then add the peroxide and expose to light and see if the number 
is increased. If it is, then some oxidisable gas will be present. 
The action of the sulphurous acid and the sunshine may, however, not be 
so simple as at first appears. There may be no S0 2 in the atmosphere 
after the products of the fires are much diluted with air. Still, there are 
great quantities of some form of sulphur products present, as all air, both 
night and day, in polluted areas gives when tested a great number of nuclei 
of spontaneous condensation, while pure air, and even pure air to which the 
products of a gas flame have been added, give none. The sun will, of course, 
act on the products of our fires in a concentrated condition while they leave 
the chimney-tops ; but as the morning fogs are fairly uniform in density, 
over considerable areas, it rather looks as if the thickening took place in the 
air which had left the chimneys some time before and become evenly 
diffused. 
Radiation Fogs. 
Before concluding, it may be as well to inquire whether there may not 
be some other possible explanation of these fogs which form at sunrise. 
For instance, may it not be possible that they are due to radiation ? It is 
well known that the radiation from the earth is stronger just after sun- 
down than it is later, owing to the air then being drier and the hazing 
effect of dust less ; and one may ask, may not the drying of the upper air 
at sunrise increase the radiation in the morning, and so cause fogging by 
the air getting cooled by radiation ? This at first sight may seem a possible 
explanation, but an examination of the morning temperatures gives no 
support to the idea. While these fogs are forming the air temperature is 
