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XI.

0N DUST AND DISEASE.

Experiments on Dusty Air.

SOLAR light in passing through a dark room reveals its
track by illuminating the dust floating in the air. “ The
sun,” says Daniel Culverwell, “ discovers atomes, though
they be invisible by candle-light, and makes them dance
naked in his beams.” 1

In my researches on the decomposition of vapors by
light, I was compelled to remove these “ atomes ” and this
dust. It was essential that the space containing the vapors
should embrace no visible thing; that no substance capable
of scattering the light in the slightest sensible degree
should, at the outset of an experiment, be found in the “ ex-
perimental tube ” traversed by the luminous beam.

For a long time I was troubled by the appearance there
of floating dust, which though invisible in diffuse daylight
was at once revealed by a powerfully-condensed beam. Two
tubes were placed in succession in the path of the air: the
one containing fragments of glass wetted with concentrated
sulphuric acid; the other, fragments of marble wetted with
a strong solution of caustic potash. To my astonishment
the dust passed through both. The air of the Royal Insti-
tution sent through these tubes at a rate sufficiently slow

1 On a day of transient shadows there is something almost magical in

the rise and dissolution of the luminous beams among the scaffolding
poles of the Royal Albert Hall.

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