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DUST AND DISEASE. 281

darkness. Without actual combustion, currents may be
generated which shall exclude the floating matter, and
therefore appear dark amid the surrounding brightness. I
noticed this effect first on placing a red-hot copper ball be--
low 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, 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 solar- spec-
trum. Right and left of this dark vertical band the float-
ing matter rose upward, bounding definitely the non-lumi-
nous 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 con-
vection current of pure air therefore passed upward among
the inert particles, dragging them after it right and left, but
forming between them an impassable black partition. This
elementary experiment enables us to render an account of
the dark currents produced by bodies at a temperature be-
low that of combustion.

When the Wire is white hot, it sends up a band of in-
tense darkness. This, I say, is due to the destruction of
the floating matter. But even when its temperature does
not exceed that of boiling water, the wire produces a dark
ascending current. This, I say, is due to the distribution
of the floating matter. Imagine the wire clasped by the

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