1. Clouds form on dust

In 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.

Clouds 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.

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