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Proceedings of the Royal Society of Edinburgh. [Sess. 
haze may also have this origin, as it forms during periods of calm weather, 
and the rising damp impure air probably causes the thickening under the 
influence of sunshine, as this thickening takes place before the air arrives 
at the cloud level. Although I have in the above generally used the word 
“ sunshine,” it must not be understood that only direct sunshine acts in 
these cases ; light through the clouds also acts, but much less powerfully, 
and only very feebly if the clouds be dense. 
It does seem very curious that we should have been seeing these fogs 
all our lives and yet should never have observed any connection between 
them and sunshine — just as with a great many other things which we see 
every day, till they become so familiar that we do not think they require 
any explanation. Now, I believe we can predict with a fair degree of 
confidence what is going to be the state of the air on any morning on which 
we have the necessary meteorological information. However clear the air 
may be before sunrise, if it be damp, the sky cloudless, and the air either 
town air or from an impure direction, then a dense haze or fog will form 
unless the wind rises. 
Though this investigation clearly shows that the sun produces certain 
kind of fogs, yet it is by no means here contended that it is to be censured 
for their appearance. It would rather appear that it is doing its best to 
show us the state of pollution into which our modern civilisation has 
brought our atmosphere, as it only inflicts these fogs on the areas upon 
which man has thrown the waste products of his industries and converted 
the atmosphere into a vast sewer, as a penalty for something wrong in his 
methods. 
The Cause of Sun-fokmed Fogs. 
As these fogs are only formed in air which has come from densely 
populated parts of the country, it seemed probable that they are formed by 
the light acting on some of the impurities thrown into the atmosphere in 
its passage over these areas. In previous papers I have shown that the 
sun is capable of forming nuclei of cloudy condensation — that is, small 
dust particles out of many different gases ; so that it seemed possible that 
the sun was here doing something of the same kind, and converting some 
of the gaseous impurities in the air from polluted districts into solid or 
liquid particles. And the question comes to be, what are the impurities in 
the polluted air out of which the sun can form nuclei ? 
But before going further it may be as well to point out that there are 
two kinds of nuclei. One kind has no affinity for water vapour and, 
though it condenses water in even unsaturated air, that action is only a 
