125 
of Edinburgh, Session 1880-81. 
cause them to condense and form nuclei on which the water vapour 
deposits. These nuclei may he solid or liquid, and may or may 
not have affinity for water vapour. 
It is concluded from the experiments, first, that as regards cloudy 
and foggy condensation there is dust and dust. Some kinds of dust 
have the power of determining condensation in an atmosphere which 
is not saturated ; other kinds only form nuclei in supersaturated air, 
and from other experiments it is probable that some degree of super- 
saturation is necessary before some other kinds of dust are active. 
In highly supersaturated air all kinds of dust will form nuclei and 
determine cloudy condensation, but in unsaturated air only some 
kinds are active. This was illustrated by corresponding phenomena 
in freezing, melting, and boiling. Second, that dry fogs may *be 
produced by some form of dust in the air, such as sodic chloride 
dust, condensing the aqueous vapour in air which is not saturated. 
Third, this condensing power or attraction which some kinds of 
dust have for aqueous vapour explains why our breath and 
condensed steam dissolve even in foggy weather. Fourth, that as 
the products of combustion of sulphur determine the condensation 
of water vapour in unsaturated air, and give use to a very fine 
textured dry fog, they are probably one of the chief causes of our 
town fogs, as they have a much greater condensing power than the 
products of combustion of coal. 
It is not claimed that these experiments prove that dry fogs in 
the country are produced by salt dust. The experiments only 
prove salt dust can produce a dry fog. As water vapour only 
condenses in some nucleus, 'it is in the highest degree probable that 
some nuclei, having strong affinities for water vapour, are the cause 
of dry fogs, and from the great amount of salt dust ever present 
in our atmosphere, it seems almost certain that it plays some part 
in the phenomena. There may be, and probably are, some other 
kinds of condensative nuclei which give rise to dry fogs in the 
country. The nature and composition of these will probably be 
best arrived at by analysis of the dried fog particles. 
There seems to be very little doubt but that sulphur products 
are most powerful fog-producers, and are probably the chief cause 
of our town fogs. Yet, it must not be forgotten that there may be 
other causes at work, of which we are at present ignorant. 
