1880 the Scottish engineer John Aitken produced small
clouds of water using a method that had been suggested
in France by Coulier and Mascart. Aitken investigated
why the clouds formed.
In his first experiment Aitken formed a cloud
by letting a jet of water vapour into a large glass
jar. If the jar contained ordinary air a cloud would
form, but if it contained filtered air no clouds appeared.
The clouds would only form if the air contained dust.
form when water vapour, which is suspended in the air, condenses
into liquid water droplets. Water vapour was condensing
onto the dust particles to form clouds. In the filtered,
dust-free air no condensation took place. The presence
of clouds showed that the air contained dust.