6. Invisible nuclei

Wilson first cleared the chamber of dust by making several expansions. Each expansion produced condensation on some of the dust in the air, which would then fall as water droplets.

But Wilson found that he could produce the drops in dust free air no matter how many times the air had already been expanded. The vapour in Wilson's experiments was forming on some sort of invisible nuclei that was being replaced.

Wilson tried a range of expansions. Larger expansions produced a larger number of smaller droplets. At certain expansions, of ratios around 1.40, the droplets were of the right size to produce the colour effects which Wilson had seen from Ben Nevis. The cloud chamber 'fogs' were producing colours in the same way that sunlight through rain can produce a rainbow.

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