15 
of Edinburgh, Session 1880-81. 
fog and cloud-particle was formerly represented by a dust-particle, 
which vapour by condensing upon it has made visible. When there 
is much dust in the air but little vapour condenses on each particle, 
and they become but little heavier, and easil} r float in the air. If 
there are few dust specks each gets more vapour, is heavier, and falls 
more quickly. 
These experiments were repeated with an air-pump, a little water 
being placed in the receiver to saturate the air. The air was then 
cooled by slightly reducing the pressure. When this is done with un- 
filtered air a dense cloudiness fills the receiver; but when with pure 
air no fogging whatever takes place, there being no nuclei on which 
the condensation can take place. In this experiment, and in the one 
with steam, the number of cloud-particles is always in proportion to 
the dust present. When the air is nearly pure and only a few dust- 
particles present, then only a few cloud-particles form, and they are 
heavy and fall like fine rain. 
The conclusions drawn from these experiments are — 1st, That 
whenever water vapour condenses in the atmosphere it always does 
so on some solid nucleus ; 2d, that dust-particles in the air form 
the nuclei on which the vapour condenses ; 3d, that if there was 
no dust there would be no fogs, no clouds, no mists, and probably 
no rain, and that the supersaturated air would convert every object 
on the surface of the earth into a condenser on which it would 
deposit ; 4th, our breath when it becomes visible on a cold morn- 
ing, and every puff of steam as it escapes into the air, show the 
impure and dusty condition of our atmosphere. 
The source of the fine atmospheric dust was then referred to, and 
it was shown that anything that broke up matter into minute parts 
would contribute a share. The spray from the ocean, when dried 
and converted into fine dust, was shown to be an important source. 
Meteoric matter also probably contributed a proportion. Attention 
was then directed to the power of heat and combustion as a source 
of this fine dust. 
It was shown that if there is much dust, then each particle only 
gets a little vapour condensed upon it, that when the particles are 
numerous they become but little heavier, and easily float in the air, 
and give rise to that close packed but light form of condensation 
which constitutes a fog, and therefore whatever increases the amount 
