f at right angles to its surface, may deposit
themselves like a very thin film on objects placed in their path.
Various physicists, among them M. Houllevigue, have studied this
phenomenon, and in the case of pressures between 1/20 and 1/100 of a
millimetre, the last-named scholar has obtained mirrors of most
metals, a phenomenon he designates by the name of ionoplasty.

But in spite of all these accessory phenomena, which even sometimes
conceal those first observed, the existence of the electron in the
cathodic flux remains the essential characteristic.

The electron can be apprehended in the cathodic ray by the study of
its essential properties; and J.J. Thomson gave great value to the
hypothesis by his measurements. At first he meant to determine the
speed of the cathode rays by direct experiment, and by observing, in a
revolving mirror, the relative displacement of two bands due to the
excitement of two fluorescent screens placed at different distances
from the cathode. But he soon perceived that the effect of the
fluorescence was not instantaneous, and that the lapse of time might
form a great source of error, and he then had recourse to indirect
methods. It is possible, by a simple calculation, to estimate the
deviations produced on the rays by a magnetic and an electric field
respectively as a function of the speed of propagation and of the
relation of the charge to the material mass of the electron. The
measurement of these deviations will then permit this speed and this
relation to be ascertained.

Other processes may be used which all give the same two quantities by
two suitably chosen measurements. Such are the radius of the curve
taken by the trajectory of the pencil in a perpendicular magnetic
field and the measure of the fall of potential under which the
discharge takes place, or the measure of the total quantity
of electricity carried in one second and the measure of the
calorific energy which may be given, during the same period, to a
thermo-electric junction. The results agree as well as can be expected,
having regard to the difficulty of the experiments; the values of the
speed agree also with those which Professor Wiechert has obtained by
direct measurement.

The speed never depends on the nature of the gas contained in the
Crookes tube, but varies with the value of the fall of potential at
the cathode. It is of the order of one tenth of the speed of light,
and it may rise as high as one third. Th