Mr. Aitken points out that if the
air were ever quite dustless, vapor could not condense, but the air
would gradually get into a horribly supersaturated condition, soaking
all our walls and clothes, dripping from every leaf, and penetrating
everywhere, instead of falling in an honest shower, against which
umbrellas and slate roofs are some protection. But let us understand
what sort of dust it is which is necessary for this condensing
process. It is not the dust and smoke of towns, it is not the dust of
a country road; all such particles as these are gross and large
compared with those which are able to act as condensers of moisture.
The fine dust of Mr. Aitken exists everywhere, even in the upper
regions of the atmosphere; many of its particles are of
ultra-microscopic fineness, one of them must exist in every raindrop,
nay, even in every spherule of a mist or cloud, but it is only
occasionally that one can find them with the microscope. It is to such
particles as these that we owe the blue of the sky, and yet they are
sufficiently gross and tangible to be capable of being filtered out of
the air by a packed mass of cotton-wool.
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