ed when they pass near the other. Experiments
bearing on this subject have been made by Crookes and Wiedemann and
Ebert. The phenomena may be described by saying that the repulsion of
the rays from a cathode A by a cathode B is only appreciable when the
rays from A pass through the Crookes dark space round B. This is what we
should expect if we remember that the electric field in the dark space
is far stronger than in the rest of the discharge, and that the gas in
the other parts of the tube is rendered a conductor by the passage
through it of the cathode rays, and therefore incapable of transmitting
electrostatic repulsion.

Scattering of the Negative Electrodes.--In addition to the cathode rays,
portions of metal start normally from the cathode and form a metallic
deposit on the walls of the tube. The amount of this deposit varies very
much with the metal. Crookes (_Proc. Roy. Soc._ 50, p. 88) found that
the quantities of metal torn from electrodes of the same size, in equal
times, by the same current, are in the order Pd, Au, Ag, Pb, Sn, Pt, Cu,
Cd, Ni, In, Fe.... In air there is very little deposit from an Al
cathode, but it is abundant in tubes filled with the monatomic gases,
mercury vapour, argon or helium. The scattering increases as the density
of the gas diminishes. The particles of metal are at low pressures
deflected by a magnet, though not nearly to the same extent as the
cathode rays. According to Grandquist, the loss of weight of the cathode
in a given time is proportional to the square of the current; it is
therefore not, like the loss of the cathode in ordinary electrolysis,
proportional to the quantity of current which passes through it.

[Illustration: FIG. 26.]

_Positive Rays or "Canalstrahlen."_--Goldstein (_Berl. Sitzungsb._ 39,
p. 691) found that with a perforated cathode certain rays occurred
behind the cathode which were not appreciably deflected by a magnet;
these he called Canalstrahlen, but we shall, for reasons which will
appear later, call them "positive rays."

Their appearance is well shown in fig. 26, taken from a paper by Wehnelt
(_Wied. Ann._ 67, p. 421) in which they are represented at B. Goldstein
found that their colour depends on the gas in which they are formed,
being gold-colour in air and nitrogen, rose-colour in hydrogen,
yellowish rose in oxygen, and greenish gray in carbonic acid.

The colour of the luminosity due to positive rays is not in general the
same as that due