Aitken then cooled the air suddenly through
expansion. Expanding the container in which the air
was held caused an increase in volume and a decrease
in the air pressure, accompanied by a drop in temperature.
In normal air this expansion caused water vapour to
condense on dust in the air, producing a cloud as before,
while in filtered air no cloud was produced.
the air had cooled the energy of the water molecules
suspended in the air had decreased. The condensation
on dust occurred because the water molecules no longer
have enough energy to float around independently.
experiments showed that expansions could produce clouds
in air which contained nucleating points, such as dust.
Air containing a little dust produced wisps of cloud,
while thick cloud could be produced in quite dusty air.