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.
