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mote-filled air. My idea is that it heats the air and lightens
it, without in the same degree lightening the floating mat-
ter. The tendency, therefore, is to start a current of clean
air through the mote-filled air. .Figure the motion of the
air all round the wire. Looking at its transverse section
we should see the air at the bottom of the wire bending
round it right and left in two branch-currents, ascend-
ing its sides and turning to fill the partial vacuum created
above the wire. Now as each new supply of air, filled with
its motes, comes in contact with the hot wire, the clean
air, as just stated, is first started through the inert motes.
They are dragged after it, but thereis a fringe of cleansed
air in advance of the motes. The two purified fringes of
the two branch-currents unite above the wire, and, keep-
ing the motes that once belonged to them right and left,
they form by their union the dark band observed in the
experiment. This process is incessant. Always the mo-
ment the mote-filled air touches the wire this distribution
is effected, a permanent dark band being thus produced.
Could the air and the particles under the wire pass through-
its mass we should have a vertical current of particles, but
no dark band. For here, though the motes would be left
behind at starting, they would hotly follow the ascending
current and thus abolish the darkness.

It has been said that when the platinum wire is intensely
heated, the floating matter is not only distributed, but de-
stroyed. Let this be proved. I stretched a wire about 4
inches long through the air of an ordinary glass shade, rest-
ing on its stand. Its lower rim rested on cotton-wool, which
also surrounded the rim. The wire was raised to a White
heat by an electric current. The air expanded, and some
of it was forced through the cotton-wool, while, when the
current was interrupted and the air Within the shade cooled,
the expelled air in its return did not carry motes along with
it. At the beginning of this experiment the shade was

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