Let me describe the appearance and
mode of seeing it by help of a diagram. (For full description see
_Philosophical Magazine_ for March, 1884.)
[Footnote 2: For instance, the electric properties of crystals
can be readily examined in illuminated dusty air; the dust grows
on them in little bushes and marks out their poles and neutral
regions, without any need for an electrometer. Magnesia smoke
answers capitally.]
Surrounding all bodies warmer than the air is a thin region free from
dust, which shows itself as a dark space when examined by looking
along a cylinder illuminated transversely, and with a dark background.
At high temperatures the coat is thick; at very low temperatures it is
absent, and dust then rapidly collects on the rod. On a warm surface
only the heavy particles are able to settle--there is evidently some
action tending to drive small bodies away. An excess of temperature of
a degree or two is sufficient to establish this dust-free coat, and it
is easy to see the dust-free plane rising from it.
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