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Proceedings of the Royal Society of Edinburgh. [Sess. 
done in order to make certain that all the conditions were the same as 
those which showed the sulphur products to be such objectionable nuclei- 
producers. 
When the results of these tests of the products of combustion of 
purified gas are compared with those obtained with sulphur, it is evident 
that, while the sulphur products give an abundance of nuclei of spontaneous 
condensation and very great quantities of nuclei without affinity for water 4 
vapour, and therefore tend to produce dense haze and fogs under the 
influence of sunshine, the products from the purified gas, on the other hand, 
as they neither form nuclei of spontaneous condensation nor other nuclei 
under the influence of sunshine, do not seem to play any part in these 
sun fogs ; though we have seen that one of the products assists in the 
formation of the nuclei formed by the S0 2 . 
Dust and Temperature. 
In a communication read before this Society on February 3, 1890, there 
is discussed the question of the relation between the amount of dust in the 
atmosphere and the temperature. It was shown that on many occasions 
when the temperature was high the number of particles was great, also 
that on these occasions the night radiation was checked, owing to the 
amount of dust in the air checking the loss of heat by radiation. It was 
shown, also, that the radiation is generally strongest just after sunset — due, 
probably, to the dust particles being dry and small at that time, and that as 
the night progresses the particles tend to grow with the increasing damp- 
ness of the air, their larger size accounting for the decrease in the radia- 
tions. It was shown that the connection between the amount of dust and 
sunshine was most marked in summer, as one might expect, owing to the 
longer hours of sunshine at that season. Then in the paper referred to the 
question is put, “ Is this increase of dust particles with sunshine a case of 
cause and effect ? or are both due to the same cause ? or is it merely a 
coincidence ? ” This investigation of the effects of sunshine on the products 
of combustion now helps us to answer these questions. It would appear 
that both the high temperature and the high dust were due to the sunshine, 
which would be abundant on warm days, the sunshine converting the 
impurities in the atmosphere into fine dust, and also heating the air, 
and the heating power of the sun would be increased by the presence 
of the high dust contents of the air, making it a better absorber of the 
heat rays. 
