car News , } als
an} Repulsion Resulting from Radiation. 245
I —————— me = ini eT ——— Fp ——— ————— | treet etn.
had none of these defects, whilst it showed the movement
T H E C H E M I C A L N E Ww S. of rotation in d very convenient manner.
145. The apparatus is shown in fig. 6. It consists of
four arms, of very fine glass, passing horizontally through
pieces of pith (6), and afterwards bent twice at right
angles, as shown in the figure. Through the centre of
the pieces of pith (b) is passed vertically the point of a
very fine sewing-needle (a), which rests in a glass cup (¢)
blown on to the end of the glass tube e. At the end of
cach glass arm is fastened a thin disk of pith, white on
one side and lampblacked on the other, the black surfaces
of all the disks {acing the same way. The whole is
:nclosed in a glass bulb blown on to the end of a wide
‘ube. fis a piece of cement to keep the support (e) in its
slace. ~ gis the tube containing cocoanut-shell charcoal ;
:he other end is sealed on to the mercury-pump, The
exhaustion is effected as already described (131); and the
ipparatus is then sealed off, with the charcoal-tube still
ittached to it. Ultimately, when all the residual gas has
been absorbed by the charcoal, a lame is applied to the
contracted part of the tube at %, and the charcoal-tube is
lisconne@ed.
146. Before adopting the above method of making these
nstruments many experiments were tried, both to secure
ase of manipulation and greater delicacy of a@ion. The
sup supporting the needle-point was made of ruby,
sapphire, chrysolite, aquamarine, and agate ; it was, how.
ever, found that these offered no advantage over glass, as
the frition was not sufficient to produce any abrasion of
the glass by the steel point. The disks at the end of the
arms were made of every imaginable substance which was
likely to answer. Among these I may mention wood,
paper, flies’ and butterflies’ wings, talc, mica, selenite,
thin glass, metals of various kinds, ivory, cork, and
pith (86). For general purposes I prefer pith, as it ig
easily cut into slices, is extremely light, dries readily in
2 vacuum, and does not evolve vapour subsequently ; besides
 which its nataral white surface i8 almost as insensi.
tive to radiation as any substance I have yet examined.
The number of disks has been varied from ten, the
maximum which can follow one another without available
surface being uselessly obscured, to two, or even one,
which latter form has been experimented with, and possesses
 some advantages, Six disks are a useful number ;
but as the difficulty of making these instruments increases
with the number of arms and disks to be got into the
bulb, I prefer four disks for ordinary purposes.
The material of which the arms are made has also been
the subject of experiment. My earlier instruments (exhibited
 at the Soirée of the Royal Society on the 7th of
April, 1875) had straw arms. These are, however, too
heavy, and are liable to evolve vapour after being kept in
the vacuum for some time. An inorganic body is preferable;
 and I have finally adopted either thin rolled brass
or fine glass thread drawn from thermometer tubing,
The colour of the disks has also been experimented on.
During this part of the inquiry many curious results have
been obtained, which will be described further on. At
present, however, I have found nothing better than lamp.
slack for the black surface and the freshly.cut pith for the
white surface. In working with cork, metals, &c., where
the natural surface is not white enough, oxide of zinc may
be used as a coating for the white surface. }
147. The lampblack is best applied to the pith surface
in the following way :—Camphor is burnt, and a sheet of
glass is held close over the flame. An abundant deposit
of lampblack takes place. A brush dipped in alcohol is
then rubbed over the deposit, and the surface is painted
over with the mixture. The lampblack adheres very well
to pith, and in a few hours the alcohol and moisture have
dried off, and a dead black, very even surface is the result,
[n some cases I smoke this again over burning camphor ;
but this is not of much use, unless the first coating shows
glistening patches or is not laid on evenly.
(To be continued.)

* A Paper communicated to the Royal Society, January g, 1876
From the Philosophical Tramsactions of the Royai Locety of London
vol. clxvl,, part 2.