only the upstreaming portion of a dust-free _coat_ perpetually being
renewed on the surface of the body. 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. The appearances may
also be examined by looking along a cylinder _toward_ the source of
light, when the dust-free spaces will appear brighter than the rest. A
rod of electric light carbon warmed and fixed horizontally across a
bell-jar full of dense smoke is very suitable for this experiment, and
by means of a lens the dust-free regions may be thus projected on to a
screen. Diminished pressure makes the coat thicker. Increased pressure
makes it thinner. In hydrogen it is thicker, and in carbonic acid
thinner, than in air. We have also succeeded in observing it in
liquids--for instance, in water holding fine rouge in suspension, the
solid body being a metal steam tube. Quantitative determinations are