366 JOHN AITKEN ON 
was noticed that when the air was nearly purified, when all the dust which had 
an affinity for vapour had received its burden of water and settled down, that 
there remained to near the end of the experiment some particles which seemed 
to require a certain degree of supersaturation before they became active. In 
highly supersaturated air all kinds of dust will form nuclei and determine 
condensation, but in unsaturated air only those kinds of dust which have an 
affinity for water will be active. We have precisely corresponding phenonema 
to this in freezing, melting, and boiling. We have water in a solid state at a 
temperature above the “ melting-point,” when it is combined with some other 
substance, as in the water of crystallisation of salts. Water may be liquid at a 
temperature below the “ freezing-point ” when mixed with some salts. Water 
boils at a temperature above its “ boiling-point ” when it holds some salts in 
solution, and boils below its “ boiling-point ” when mixed with some substance 
having a lower “ boiling-point ” than water. 
2. This affinity which some kinds of dust have for vapour explains why it 
is that our breath and escaping steam dissolve even in foggy air. The large 
cloudy particles in our breath and in condensed steam tend to evaporate in the 
same air in which condensation is taking place, because the dust particles on 
which the breath has condensed have had their affinities more than satisfied, they 
therefore tend to part with their surplus by evaporisation in the same air as those 
particles which have not had their affinities satisfied tend to condense it. 
3. Dry fogs are produced by the affinity which the dust particles have 
for water vapour, in virtue of which they are enabled to condense vapour in 
unsaturated air. From the experiments with chloride of sodium, from the 
known affinity of that salt for water, and from the fact that great quantities of 
salt-dust are ever present in the air, it is evident that if it is not the cause of 
dry fogs in the country it must play some part in those phenomena. There will 
doubtless be other kinds of nuclei having affinities for water which will cause dry 
fogs. The nature and composition of these other nuclei will probably be best 
arrived at by collecting the fog particles by washing or otherwise, and analysing 
them. 
4. That as the products of combustion of the sulphur in our coals, espe- 
cially when mixed with the other products of combustion, such as ammonia, 
have the power of determining the condensation of water vapour in unsaturated 
air, and give rise to a very fine-textured dry fog, they are probably one of 
the chief causes of our town fogs, as they have a greater condensing power than 
the products of combustion of pure coal. 
Though there may seem to be but little doubt that products of combus- 
tion when mixed with the sulphur compounds are most active producers 
of town fogs, yet we must not rest satisfied that they explain everything. 
There may be other causes at work, and conditions yet requiring explanation, 
