began by making a study of Aitken's method for producing
clouds. Wilson recorded the amount by which he expanded
the air in the glass chamber and tried larger expansions
than Aitken had performed. By expanding the volume enough,
Wilson made an unexpected discovery.
the expansion ratio was just over 1.25, meaning the
volume of the chamber was made a quarter as big again,
Wilson found drops of condensation formed even if the
air was free of dust. Aitken had been unable to produce
drops in dust free air.