CHEMICAL News, |
July 13, 1877.

Repulsion Resulting from Radiation. 13
can vary at will. The bulb was perfectly exhausted, and
the following experiments were tried :—
176. The resistance.coil was so adjusted that the
battery would keep the platinum spiral (c) at a bright red
heat. The arms of the radiometer, which were before
quite still, moved rapidly until two of the disks wers one
on each side of the hot spiral, the black disk being further
off than the white disk as shown at i. The resistance
was then gradually increased, and as the temperature of
the spiral diminished, the black disk gradually approached
the spiral, until, when the temperature was just at the
point of visible redness in a dark room, the black and
white disks were practically equidistant from the spiral.
On diminishing the resistance, the same phenomena took
place in inverse order.
Fie. 10.

THE CHEMICAL NEWS.

VoL. XXXVI. No. 020.

ON REPULSION RESULTING FROM
RADIATION.—PART IIL*
8y WILLIAM CROOKES, F.R.S., &c.
{Continued from p. 2.)

t72. LET me now carry the reasoning a step further.
Experiments described in pars. 159 to 168 show thal
when heat of low refrangibility—from the body (159), the
breath (160), hot air, or a warm glass shade (161, 165.
168)—falls on the white and black surfaces, the white is
repelled more than the black, rotation of the radiometer
taking place in the negative direion. The same rays
falling on the two surfaces do more work on the white
than on the black ; and this, to my mind, appears sufficient
 to make it almost certain that the white pith absorbs
more of these low rays than does the lampblack.
173. Let us imagine that surfaces of lampblack and pith
are carried along the sperum from the blue to the ultra.
red. As long as they are in the visible portion we observe
an enormous difference between them. In the extreme
red we can actually see that this difference is becoming
less. In my mind's eye I pi@ure the progress being continued
 along the whole length of the ultra-red spectrum.
I can see, by the light of the above-quoted experiments,
that the absorptive action of the two surfaces gradually
gets more equal. Soon they become identical in their
action on the incident rays, and after that they enter a
portion of the spectrum whose rays are no longer absorbed
by lampblack, whilst they are quenched by the pith.
Lampblack and pith have now changed places ; the latter
is black, whilst lampblack has become a white substance.
474. The normal rotation of the radiometer caused by
dropping ether on it (163) is perfectly well explained by
the above hypothesis. If heat in the a@ of absorption
produces motion in one dire@ion, in the ac of radiation it
produces motion in the opposite direction (167). Heat of
low refrangibility falling on the radiometer repeis the
white more than it does the black, and produces negative
rotation. When the same kind of heat is drawn out of
the black and white surfaces by the chilling action of the
ether, movement takes place in an opposite direction, and
the arms rotate normally. Un stopping this efflux of heat
by covering the instrument with a hot shade (163), I
changed the direction of movement by causing the surfaces
 to absorb instead of emit heat.
An irregular emission or absorption of heat (164) stops
the movement altogether, for the reasons given in pars.
155 to 158.
175. 1 have made an apparatus by means of which I
hoped to put the above theory to accurate test. The
results are not so definite as they ought to be in order to
settle the question ; but they are worth giving in detail,
as some novel fa@s have been elicited by them.
Fig. 10 shows the instrument; a is the bulb of a radiometer
 of the usual construction, having pith disks blacked
on one side. bis a tube sealed into one side of the bulb,
and having two stout platinum wires passing along it,
sealed their whole length in glass to prevent leakage of
zir into the interior of the apparatus. At the ends ¢ of
the wires, a spiral of fine platinum wire is fastened, and
the other ends (d) terminate in loops outside. ¢ is a
oattery, f a contact-kev, and g a resistance.coil which 1

5

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7

177. The resistance was adjusted to give a bright red
ipiral, and the conta&-key kept pressed down. The disks
stood as at i. A lighted match was momentarily brought
near the bulb, so as to start a movement. Rotation of
the arms commenced, and kept up, with some energy, at
the rate of about 1 revolution in five seconds, equal to
that given by a candle 8 inches off. There was some
little hesitation as the white side came up to the spiral,
but this was scarcely noticed when the speed had become
steady.
The resistance was slightly increased. The speed
became slower as the temperature of the spiral diminished,
and the hesitation as the white approached the spiral
became more apparent. The resistance was further increased,
 with the effect of making rotation still slower,
I now brought the temperature of the spiral dewn to
just visible redness in the dark. The speed of rotation
again slackened ; at each approach of the white surface
to the spiral it stopped, hesitated, and then got past with
a rush. Thus it went on for a few revolutions, until one
white disk, a little nearer perhaps than the others, was
not able to pass, and the arms after a few oscillations
came to rest, the black and white surfaces being, as near
as I could judge, eqidistant from the hot spiral.
These results fully confirm those obtained in experiments
128 and 144, and I think justify the conclusions arrived
at in my discussion of them at par. 171—that at temperatures
 between 250° and 100° the repellent action of
radiant heat is about equal on black and on white surfaces,
178. I now wish to ascertain whether the continuation
of the reasoning (172) was corre@—whether at tempera.
tures lower than 100° C. the white would be repelled
most.

* A Paper communicated to the Royal Society, January 5, 1876
from the Philosophical Transactions of the Royal Society of London
rol. clxvi.. part 2.