this matter, rose into the beam, jostled aside
the illuminated particles, and substituted for their light the
darkness due to its own perfect transparency. Nothing could more
forcibly illustrate the invisibility of the agent which renders all
things visible. The beam crossed, unseen, the black chasm formed by
the transparent air, while, at both sides of the gap, the thick-strewn
particles shone out like a luminous solid under the powerful
illumination.

It is not, however, necessary to burn the particles to produce a
stream of darkness. Without actual combustion, currents may be
generated which shall displace the floating matter, and appear dark
amid the surrounding brightness. I noticed this effect first on
placing a red-hot copper ball below the beam, and permitting it to
remain there until its temperature had fallen below that of boiling
water. The dark currents, though much enfeebled, were still produced.
They may also be produced by a flask filled with hot water.

To study this effect a platinum wire was stretched across the beam,
the two ends of the wire being connected with the two poles of a
voltaic battery. To regulate the strength of the current a rheostat
was placed in the circuit. Beginning with a feeble current the
temperature of the wire was gradually augmented; but long before it
reached the heat of ignition, a flat stream of air rose from it, which
when looked at edgeways appeared darker and sharper than one of the
blackest lines of Fraunhofer in the purified spectrum. Right and left
of this dark vertical band the floating matter rose upwards, bounding
definitely the non-luminous stream of air. What is the explanation?
Simply this: The hot wire rarefied the air in contact with it, but it
did not equally lighten the floating matter. The convection current
of pure air therefore passed upwards among the inert particles,
dragging them after it right and left, but forming between them an
impassable black partition. This elementary experiments enables us to
render an account of the dark currents produced by bodies at a
temperature below that of combustion.

But when the platinum wire is intensely heated, the floating matter is
not only displaced, but destroyed. I stretched a wire about 4 inches
long through the air of an ordinary glass shade resting on
cotton-wool, which also surrounded the rim. The wire being raised to
a white heat by an electric current, the air expanded, and some of it
was