5. Clouds without dust!

Wilson 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.

When 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.

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