DUST, FOGS, AND CLOUDS. 367 
but as these involve intricate chemical reactions, it will be advisable that the 
matter be now handed over to the consideration of the chemist. 
These chemical nuclei, as they might be called, though found in far greatest 
abundance in the air of our towns, will no doubt be also found in the air of 
the country. We know that sulphuric acid and ammonia are constantly being 
produced by decomposing animal and vegetable matter, and we know that 
these substances, along with nitric acid and other gases and vapours, are 
always present in the air. 
Again, we have the gases given off from volcanoes, and the amount from 
this source must be considerable. There are about two hundred active vol- 
canoes constantly discharging their gases into our atmosphere, and it has been 
roughly calculated that volcanoes evolve ten times more carbonic acid than is 
given off by the combustion of all kinds of carbonised material. With this 
carbonic acid there is given off great quantities of sulphurous and other gases 
which will condense and form nuclei. 
Vegetation, both when alive and when dead, gives off vast quantities of 
small organic particles, and microscopic life, which almost seem to populate 
the air we breath, and will of course add much to the dust in our atmosphere. 
Professor TyNpALL has shown that light decomposes certain gases and 
vapours, and that this decomposition is greatly aided by the presence of 
other gases or vapours. It seems therefore probable that the sun’s rays will 
decompose some of the gases and vapours in the air, and if these decomposed 
substances have a lower vapour tension than the substance from which they 
are formed, they condense into very fine particles. These particles may be 
solid or liquid, and will form nuclei for the condensation of water vapour. 
We know that there are ever present in our atmosphere great quantities of 
chloride of sodium and other kinds of dust which have affinities for water. 
These dust particles by their affinities for water vapour cause condensation 
to take place in unsaturated air, and if present in great quantities give rise 
to dry fogs. “Let us look briefly at the effect of this affinity between the dust 
and the vapour. If there was no affinity between the two, then condensation 
would only begin when supersaturation began, and those dust particles which 
permitted the vapour to condense on them easiest would get most vapour, and 
would tend to grow largest. This would evidently tend to inequality in the 
size of the cloud particles which would determine the fall of some of them 
through the others. But if there is an affinity between the dust and the vapour, 
then each particle of dust tends to take the same amount of vapour, and if one 
particle gets more than its proportion, the others tend to rob it of its surplus. 
This evidently tends to equality in the size of the cloud particles, and tends 
also to prevent any of them falling through the others, and thus prevents it 
beginning to rain, that is, if rain drops are formed by the collision and union of 
