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But if this production of colour and beauty were the only useful
function of dust, some persons might be disposed to dispense with it in
order to escape its less agreeable effects. It has, however, been
recently discovered that dust has another part to play in nature; a part
so important that it is doubtful whether we could even live without it.
To the presence of dust in the higher atmosphere we owe the formation of
mists, clouds, and gentle beneficial rains, instead of water spouts and
destructive torrents.
It is barely twenty years ago since the discovery was made, first in
France by Coulier and Mascart, but more thoroughly worked out by Mr.
John Aitken in 1880. He found that if a jet of steam is admitted into
two large glass receivers,--one filled with ordinary air, the other with
air which has been filtered through cotton wool so as to keep back all
particles of solid matter,--the first will be instantly filled with
condensed vapour in the usual cloudy form, while the other vessel will
remain quite transparent.


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