ese X rays may give birth to the secondary rays of M.
Sagnac; and often we find ourselves in presence of effects from these
last-named radiations and not from the true cathode rays.

The electrons, when they are propagated in a gas, can ionise the
molecules of this gas and unite with the neutral atoms to form
negative ions, while positive ions also appear. There are likewise
produced, at the expense of the gas still subsisting after
rarefication within the tube, positive ions which, attracted by the
cathode and reaching it, are not all neutralised by the negative
electrons, and can, if the cathode be perforated, pass through it, and
if not, pass round it. We have then what are called the canal rays of
Goldstein, which are deviated by an electric or magnetic field in a
contrary direction to the cathode rays; but, being larger, give weak
deviations or may even remain undeviated through losing their charge
when passing through the cathode.

It may also be the parts of the walls at a distance from the cathode
which send a positive rush to the latter, by a similar mechanism. It
may be, again, that in certain regions of the tube cathode rays are
met with diffused by some solid object, without having thereby changed
their nature. All these complexities have been cleared up by M.
Villard, who has published, on these questions, some remarkably
ingenious and particularly careful experiments.

M. Villard has also studied the phenomena of the coiling of the rays
in a field, as already pointed out by Hittorf and Pluecker. When a
magnetic field acts on the cathode particle, the latter follows a
trajectory, generally helicoidal, which is anticipated by the theory.
We here have to do with a question of ballistics, and experiments duly
confirm the anticipations of the calculation. Nevertheless, rather
singular phenomena appear in the case of certain values of the field,
and these phenomena, dimly seen by Pluecker and Birkeland, have been
the object of experiments by M. Villard. The two faces of the cathode
seem to emit rays which are deviated in a direction perpendicular to
the lines of force by an electric field, and do not seem to be
electrified. M. Villard calls them magneto-cathode rays, and according
to M. Fortin these rays may be ordinary cathode rays, but of very
slight velocity.

In certain cases the cathode itself may be superficially
disaggregated, and extremely tenuous particles detach themselves,
which, being carried of