Cops Nova, | Repulsion Resulting from Radiation.
THE CHEMICAL NEWS.

267%
ment. But, notwithstanding these drawbacks, I think
the radiometer is likely to bes a more popular form
af light-measurer. It requires no adjustment, and is
always ready to be observed, whilst there is a peculiar
charm in using an instrument which is constantly in
active work, With the exception of the comparison by
balancing ene light against another, all the observations
mentioned in paragraph 140 can be taken with the ra fiometer,
 and it is besides capable of applications of its
own. I will mention one, although others easily suggest
‘hemselves,
As theradiometer will revolve behind the orangecoloured
glass used by photographers for admitting light into their
so-called dark-room, it is only necessary to have one of
these instruments in the window to enable the op -rator
to see whether the light entering his room is likely to
injure the sensitive surfaces there exposed; thus, having
ascertained by experience that his plates are fogged or
his paper injured when the revolutions exceed, say, one
in three seconds, he will take care to draw down an extra
olind when the revolutions approach that number. In
shotographic operations a radiometer may be placed in
jume conven ent spot near the objet to be copied.
Having ascertained, once for all, how manyrevolutions the
‘nstrument makes whilst a good negative is being taken, the
operator need in future take ns account of the variation
of light, but simply expose for the same number of revo.
lutions, with a certainty that his negatives wii all be of
the same quality.
For the more important work of gas-testing probably
‘he bar-instrument already described (135) will be more
valuable ; although, even for this purpose, the radiometer
will he found to give very rapid and trustworthy indications.

155. I have already mentioned that the motion of the
radiometer depends on a differential a&ion of radiation
on the black and white surfaces. To obtain rotation in
the ordinary way the black must be repelled with more
energy than the white; and this appears to be the case
with all the luminous rays. In the case of dark heat,
however, this difference of aion is not apparent (128).
The following experiments were tried with various radio.
meters :—
A candle was placed at such a distance from a radiometer
 that the fly would make one revolution a minute,
A small glass flask of boiling water was then placed half
an inch from the bulb. The revolutions instantly stopped,
two of the arms setting equi-distant from the hot-water
flask, The candle was kept in the same position, and
the flask of water was removed, As the portion of the
bulb which had been heated by the hot water cooled, the
white surface gradually crept nearer and nearer to it, the
superior repulsion of the candle on the black disks urging
the arms round and a@ing in opposition to the repulsion
of the hot glass to the white disk. At last the force of
the light drove the white disk with difficulty past the hot
spot of glass. Rotation then commenced, but for some
revolutions there appaared to be a difficulty in the white
disks passing the spot of glass which had been warmed
by the hot water.
156. The flask of boiling water was then replaced in
its position half an inch from the bulb of the radiometer.
The rotation immediately stopped. The candle was
ihen brought gradually nearer and nearer to the instru.
ment, but with no particular effect. As it came very near
the arms vibrated to and fro, and appeared to make
violent efforts to get round, but no force of the light
:eemed sufficient to overcome the repugnance of the
white disk to pass the heated portion of the glass.
157. The radiometer was allowed to cool, and the
candle was again placed in the first position, where it
produced one revolution in a minute. The finger was
pressed against the side of the bulb, As the spot of
glass got warm the white surface experienced more and
more difficulty in getting past it, until at last one disk refused
 to pass, and the arms came to rest.

Var. XXXV. No. g18.

ON REPULSION RESULTING FROM
RADIATION.—PART II1.*
By WILLIAM CROOKES, F.R.S,, &c.
(Continued from p. 256.)

152. THE following experiments were tried with a very
sensitive radiometer in a 2-inch bulb. The moving part
(the “ fly”), consisting of glass arms, pith disks, and steel
point, weighed only 0'8 grain. It was exhausted with a
charcoal reservoir attached (131).
A standard candle, placed 2 inches from the centre,
made the arms spin with a velocity of 4 revolutions per
second ; with the candle 4 inches off the velocity was
10 revolutions in 11 seconds. In the full sunshine of a
November day the speed was too great to count.
Nothing was visible but an undefined nebulous ring,
which became more or less distin@ as the sunlight increased
 and diminished owing to passing clouds. This
speed was kept up for more than an hour; indeed there
appears no reason why it should ever diminish as long
as light of uniform intensity shines on it.
153. The same radiometer was tried by the light of a
candle, 4 inches off, behind different screens, with the
following results :—
1 Candle, 4 nches off:

Seconds for
t Revolution.

Naked flame .. .. ,. 1
Behind thin white glass 3
thick plate glass Los
purple glass. I;
dark red glass .. 5
pink glass... .. 6
light yellow glass 2'1
blue glass .. .. . 23
orange glass .. ., oo2v
green glass EE EX
“eclipse” glasses (blue and orange, | .
almost opaque to daylight RA A
7} millims. of waterincell .. .. .. 60
solution chromate of potash, 7} wilings. | Go
thick... .. .. .. 0. ..
sol. bichromate of potash, 74 m.m. thick 7'0
clear plate of alum, 5 m.m. thick.,. .. 9'0
sol. chloride of cobalt, 74 m.m. thick .. 12%
sol. ferrocyanide of potassium, 74 m.m. 6
thick .. .. .. .. .  .. ..}7060
sol. ammonio-sulphate of co er, |
7% m.m, thick hn we we reer } 200
sol. ferricyanide of potassium, 74 m.m. }
thick ae oo Lom TA ..} 300
sol. sulphate of nickel, 74 m.m. thick.. 35'0
sol. sulphate of copper, 74 m.m. thick. 390
154. The instrument is thus seen to be capable of
very extended use as a measurer of radiation of any desired
 kind, Unlike the instrument described in paragraph
135, it cannot be used for acually balancing one quality
of light against another; and the method of taking an
observation is not ro accurate, for it is less easy to count
revolutions per second or per minute than to observe the
movement of a spot of light along a graduated scale.
There are besides many causes which tend to interfere with
the accuracy of the ‘indications of this form of instru-*

 A Paper communicated to the Royal Society, January 5, 1876
From the Philosophical Transactions of the Royal Soctety of London,
vol. clxvi., part 2.