king the sides of the glass tube.
The composition of the glass seemed to enter into this phosphorescence
also, for while lead glass produced blue phosphorescence, soda glass
produced a yellowish green. The composition of the glass seemed to
be changed by a long-continued pelting of these particles, the
phosphorescence after a time losing its initial brilliancy, caused by
the glass becoming "tired," as Professor Crookes said. Thus when some
opaque substance, such as iron, is placed between the cathode and the
sides of the glass tube so that it casts a shadow in a certain spot
on the glass for some little time, it is found on removing the opaque
substance or changing its position that the area of glass at first
covered by the shadow now responded to the rays in a different manner
from the surrounding glass.

The peculiar ray's, now known as the cathode rays, not only cast a
shadow, but are deflected by a magnet, so that the position of the
phosphorescence on the sides of the tube may be altered by the proximity
of a powerful magnet. From this it would seem that the rays are composed
of particles charged with negative electricity, and Professor J. J.
Thomson has modified the experiment of Perrin to show that negative
electricity is actually associated with the rays. There is reason for
believing, therefore, that the cathode rays are rapidly moving charges
of negative electricity. It is possible, also, to determine the velocity
at which these particles are moving by measuring the deflection produced
by the magnetic field.

From the fact that opaque substances cast a shadow in these rays it was
thought at first that all solids were absolutely opaque to them. Hertz,
however, discovered that a small amount of phosphorescence occurred on
the glass even when such opaque substances as gold-leaf or aluminium
foil were interposed between the cathode and the sides of the tube.
Shortly afterwards Lenard discovered that the cathode rays can be made
to pass from the inside of a discharge tube to the outside air. For
convenience these rays outside the tube have since been known as "Lenard
rays."

In the closing days of December, 1895, Professor Wilhelm Konrad
Roentgen, of Wurzburg, announced that he had made the discovery of the
remarkable effect arising from the cathode rays to which reference
was made above. He found that if a plate covered with a phosphorescent
substance is placed near a discharge tube exhausted so highly that the