tisfied with
these explanations, and repeated the experiment very carefully. He
noted several new points, and hit on the capital idea of seeing what a
cold body did. From the cold body the descending current was just as
dark and dust-free as from a warm body. Combustion and evaporation
explanations suffered their death-blow. But he was unable to suggest
any other explanation in their room, and so the phenomenon remained
curious and unexplained.

In this state Mr. Clark and I took the matter up last summer, and
critically examined all sorts of hypotheses that suggested themselves,
Mr. Clark following up the phenomena experimentally with great
ingenuity and perseverance. One hypothesis after another suggested
itself, seemed hopeful for a time, but ultimately had to be discarded.
Some died quickly, others lingered long. In the examination of one
electrical hypothesis which suggested itself we came across various
curious phenomena which we hope still to follow up.[2] It was some
months before what we now believe to be the true explanation began to
dawn upon us. Meanwhile we had acquired various new facts, and first
and foremost we found that the dark plane rising from a warm body was
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-fre