512 
ME. W. CEOOKES ON ATTEACTION AND 
sion of the pith ball takes place in a perfectly regular manner ; but if the glass rod has 
been passed once or twice through the fingers, or if it is rubbed a few times sideways 
along the exhausted tube, the beam immediately moves about in a very irregular 
manner, sometimes being repelled from and at others attracted to the side of the tube, 
where it sticks until the electrical excitement subsides. When the finger is rubbed 
against the exhausted glass tube, the same electrical interference takes place ; attrac- 
tions and repulsions occur by fits and starts, the pith sticks to the tube, and does not 
regain its ordinary state for some hours. When a small spirit-flame is passed beneath 
the pith end of the balance in the vacuum, a similar but much fainter electrical effect is 
noticed. This, however, is not sufficient to interfere with the repulsion due to radiation 
unless the vacuum is very good and free from aqueous vapour, 
35. The end of a glass beam in a Sprengel vacuum was found to be attracted by 
either pole of an induction-coil, when the other pole was not well insulated. 
To ascertain whether electricity exerted any special action in the ordinary repulsions 
in vacuum, the following experiments were tried : — 
36. A straw beam with pith extremities was enclosed in a tube (24) and exhausted 
to the full power of the Sprengel pump. After adjustment by heat, till it was in 
equilibrium and very delicate to slight radiation, it was re-exhausted and hermetically 
sealed up. The tube was then completely surrounded with wet blotting-paper, with 
the exception of a small aperture through which the movement of the beam could be 
observed ; the blotting-paper was connected to earth by a wire soldered on to a gas-pipe. 
On bringing a piece of warm copper beneath the pith ball, it rose as readily as if 
the outside of the tube had been dry and insulated. The finger moistened with warm 
water also repelled the pith ; and when cooled with melting ice, and then applied 
dripping wet either above or below the pith ball, there was attraction. The same 
result, but more strongly marked, took place when a piece of ice was used instead of 
the cold finger. 
A straw beam furnished with brass balls at each end* was suspended in the usual 
manner on a double-pointed needle; and the brass balls and needle were placed in 
metallic connexion by means of a very fine platinum wire. The needle did not rest 
on the sides of the*glass tube but in steel cups, to which w T as soldered a platinum wire 
passing through the glass tube and connected to earth. The tube was then exhausted, 
and the usual experiments tried with hot and cold bodies, both with and without a wet 
blotting-paper cover. In all cases the brass balls behaved normally, being repelled by 
heat and attracted by cold. 
These experiments show that electricity is not a chief agent in these attractions and 
repulsions, however much it may sometimes interfere with and complicate the pheno- 
mena. 
* Preliminary experiments showed me that the brass balls on the straw beam acted in every respect the 
same as the pith balls, with regard to hot and cold bodies externally applied, both at ordinary pressures and in 
a vacuum ; but they moved in a more sluggish manner. 
