of Edinburgh, Session 1880 - 81 . 
17 
introduced into the experimental receiver and mixed with steam, 
and the relative densities of the fog produced were noted. It 
was found that the air of the laboratory where gas was burn- 
ing always gave a denser fog than the air outside, and that the 
air outside varied, giving less fog during wet than during dry 
weather. The products of combustion of gas burned in a Bunsen 
flame, a bright flame, and a smoky flame, were all tested and found 
to be about equally bad, and all much worse than the air in which 
they were burned. Products of combustion from a clear fire and 
from a smoky one gave about equal fogging, and both much worse 
than the air of the room. 
Experiments were made by burning different substances. Com- 
mon salt when burned in a fire or in alcohol flame gave an 
intensely fog-producing atmosphere, but burned sulphur was the 
most active substance experimented on. It gave rise to a fog so 
dense it was impossible to see through a thickness of 5 cm. of it. 
The vapours of other substances than water were tested to see 
if they would condense in the cloud form without nuclei on which 
to deposit. All the substances experimented on, which included 
sulphuric acid, alcohol, benzole, and paraffin, only gave a cloudy 
condensation when mixed with ordinary unfiltered air, and remained 
perfectly clear when mixed with filtered air, all these acting like 
water vapour. 
Before referring to fogs, which have now become so frequent and 
aggravated in our large towns, it was pointed out that caution was 
necessary in applying the results of the experiments. 
The conditions of a laboratory experiment are so different, and on 
so small a scale, that it is not safe to carry their teaching to the utmost 
limits, and apply them to the processes which go on in nature. We 
may, however, look to the experiments for facts from which to reason, 
and for processes which will enable us to understand the grander 
workings of nature. 
It having been shown that vapour, by condensing on the dust- 
particles in the air, gives rise to a fogging, the density of which 
depends on the amount of fine dust in the air, the more dust the 
finer are the fog-particles, and the longer they remain suspended in 
the air ; It having been also shown that all forms of combustion, 
perfect and imperfect, are producers of fpg nuclei, it is concluded 
VOL. XI. 
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