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284 FRAGMENTS OF SCIENCE.

4. The experiment was repeated, with the difference
that the air was sent more slowly through the red-hot tube.
The floating matter was-totally destroyed.

5. The platinum tube was now lowered until it bordered
upon a visible red heat. The air sent through it still more
slowly than in the last experiment carried with it a cloud
of floating matter.

If, then, the suspended matter is destroyed by abright-
red heat, much more is it destroyed by a flame, whose tem-
perature is vastly higher than any here employed. So that
the blackness introduced into a luminous beam where a
flame is placed beneath it is due, as stated, to the destruca
tion of the suspended matter. At a dull-red heat, how-
ever, and still more when only on the verge of redness, the
platinum tube permitted the motes to pass freely. In the
latter case the temperature was 800° or 900° Fahrenheit.
This was unable to destroy the suspended matter; much
less, therefore, would a platinum wire heated to 212° be
competent to do so. Such a wire can only distribute the
matter, not destroy it.

The floating dust is revealed by intense local illumina-
tion. It is seen by contrast with the adjacent illuminated
space; the brighter the illumination the more sensible is
the difference. N ow, the beam employed in the foregoing
experiments is not of the same brightness throughout its
entire transverse section. Pass a white switch, or an ivory
paper-cutter, rapidly across the beam, the impression of its
section will linger on the retina. The section seems to float
for a moment in the air as a luminous circle, with a rim much
brighter than its central portion. The core of the beam is
thus seen to be enclosed by an intensely-luminous sheath.
An effect complementary to this is observed when the beam
is intersected by the dark band from the platinum wire.
The brighter the illumination the greater must be the rela-
tive darkness consequent on the withdrawal of the light.

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