500 
NATURE 
[March 17, 1870 
But here a difficulty meets us. It is not necessary to burn the 
particles to produce a stream of 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 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 aug- 
mented, 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 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 particles, dragging 
them after it right and left, but forming between them an im- 
passable black partition. In this way we render an account of 
the dark currents produced by bodies at a temperature below 
that of combustion. 
This explanation has been found difficult. When the 
wire is white hot, it sends up a band of intense darkness. 
This, I say, is due tothe 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 déstizbution of the floating matter. 
The difficulty alluded to is probably to be referred to the 
brevity of the explanation. Imagine the wire clasped by 
the mote-filled air. My idea is that it heats the air and 
lightens it, without in the same degree lightening the 
floating matter. 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 it 
transversely we should see the air at the bottom of the 
wire bending round it right and left in two branch cur- 
rents, ascending 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 there is a 
fringe of cleansed air in advance of the motes. The two 
purified fringes of the two branch currents unite above 
the wire, and, keeping 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 in- 
cessant. Always.the moment 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 ¢hrough 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 
destroyed. Let this be proved. I stretched a wire about 
four inches long through the air of an ordinary glass-shade 
resting on its stand. Its lower rim rested on cotton wool, 
which also surrounded the rim. The wire was raised toa 
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 charged with floating matter; at the end of 
half an hour it was optically empty. 
On the wooden base of a cubical glass shade measuring 
eleven and a half inches a side, upright supports were 
fixed, and from one support to the other thirty-eight 
inches of platinum wire were stretched in four parallel 
lines. The ends of the platinum wire were soldered to 
two stout copper wires which passed through the base of 
the shade and could be connected with a battery. As 
in the last experiment the shade rested upon cctton 
wool A beam sent through the shade revealed the 
suspended matter. The platinum wire was then raised 
to whiteness. In five minutes there was a sensible 
diminution of the matter, and in ten minutes it was 
totally consumed. This proves that when the platinum 
wire is sufficiently heated, the floating matter, instead 
of being distributed, is destroyed. 
But is not the matter really of a character which 
permits of its destruction by the moderately heated 
platinum wire also? Here is the reply:— 
1. A platinum tube with its plug of platinum gauze was 
connected with an experimental tube, through which a 
powerful beam could be sent from an electric lamp placed 
at its end. ‘The platinum tube was heated till it glowed 
fecbly but distinctly in the dark. The experimental tube 
was exhausted and then filled with air which had passed 
through. the red-hot tube. A considerable amount of 
floating matter which had escaped combustion was 
revealed by the electric beam. 
2. The tube was raised to brighter redness and the air 
permitted to pass slowly through it. Though diminished 
in quantity, a certain amount of floating matter passed 
into the exhausted experimental tube. 
3. The platinum tube was rendered still hotter; a 
barely perceptible trace of the floating matter now passed 
through it. 
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 a bright 
red heat, much more is it destroyed by a flame whose 
temperature 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 
destruction of the suspended matter. Ata dull red heat, 
however, 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 goo’ 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. Itis seen by contrast with the adjacent unillumi- 
nated space; the brighter the illumination the more 
sensible is the difference. Now the beam employed in 
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