20 7 
1911-12.] The Sun as a Fog Producer. 
tests made on the products of burning coal. In these experiments it was 
seen that a very small amount of the products provided material for the 
production of great quantities of fine dust under the influence of sunshine. 
The quantity of matter required is evidently extremely small when we 
consider that a filter which has once been used to filter the products of the 
combustion of coal will afterwards give off enough material to the air 
passing through it to cause fogging after sunning in a very great quantity 
of air. The hazing effect of these very small particles is probably mainly 
due to the water condensed on them by surface action. 
There is a point which I fear may have been lost sight of, and which it 
is very necessary should be kept in mind — namely, that in these experiments 
with the products from coal the question of smoke is entirely eliminated, 
as the products were filtered. Here, therefore, comes an important conclusion 
— namely, that all the nuclei produced by sunshine acting on the im- 
purities will still come into existence and haze and fog the atmosphere, 
even though our laws succeed in stopping all smoky chimneys. 
In addition to the impurities thrown into the atmosphere in this 
country, we must remember that enormous quantities are also added by our 
neighbours across the seas, with the result that the air coming from them, 
with winds blowing from east, south-east, and south, comes to us already 
much polluted. East winds coming to the east coast of Scotland are 
always highly charged with fine dust, and, unless when very dry, are 
heavily hazed when they arrive on our shores. The south winds, which 
are more impure than the east on the east coast, have their impurities 
mainly from our own fires, though they are far from being pure on their 
arrival on our southern shores. The only really pure air that comes to 
this country is that from the Atlantic on our west and north coasts. 
At Falkirk there are no sun fogs when the wind is from the north-west 
quadrant, because the air from that direction is pure and almost free from 
the products of combustion ; hut it seems probable that at many places in 
this country, especially inland ones, sun fogs will be produced with winds 
from all directions, as they bring with them the products of combustion 
from the fires and furnaces in every direction. 
Products of Combustion of Purified Gases. 
Having shown that the products of combustion of sulphur, whether 
burned alone or in the combustion of coal, give rise to very great numbers 
of nuclei, and also produce nuclei which condense water in unsaturated air, 
we will now turn to the consideration of the products of the other con- 
