spond more vigorously to the stimulus than those portions which have not
been protected. Skinner (_Proc. Camb. Phil. Soc._ ix. p. 371) and Thomson
found on the glass which had been exposed to the rays gelatinous
filaments, apparently silica, resulting from the reduction of the glass. A
reducing action was also noticed by Villard (_Journ. de phys._ 3, viii. p.
140) and Wehnelt (_Wied. Ann._ 67, p. 421). It can be well shown by
letting the rays fall on a plate of oxidized copper, when the part struck
by the rays will become bright. The rays heat bodies on which they fall,
and if they are concentrated by using as a cathode a portion of a
spherical surface, the heat at the centre becomes so great that a piece of
platinum wire can be melted or a diamond charred. Measurements of the
heating effects of the rays have been made by Thomson (_Phil. Mag._ [5],
44, p. 293) and Cady (_Ann. der Phys._ 1, p. 678). Crookes (_Phil.
Trans._, 1879, pt. i. p. 152) showed that a vane mounted as in a
radiometer is set in rotation by the rays, the direction of the rotation
being the same as would be produced by a stream of particles proceeding
from the cathode. The movement is not due to the momentum imparted to the
vanes by the rays, but to the difference in temperature between the sides
of the vanes, the rays making the side against which they strike hotter
than the other.

[Illustration: FIG. 23.]

_Effect of a Magnet._--The rays are deflected by a magnet, so that the
distribution of phosphorescence over the glass and the shape and
position of the shadows cast by bodies in the tube are altered by the
proximity of a magnet. The laws of magnetic deflection of these rays
have been investigated by Plucker (_Pogg._ _Ann._ 103, p. 88), Hittorf
(_Pogg. Ann._ 136, p. 213), Crookes (_Phil. Trans._, 1879, pt. 1, p.
557), and Schuster (_Proc. Roy. Soc._ 47, p. 526). The deflection is the
same as that of negatively electrified particles travelling along the
path of the rays. Such particles would in a magnetic field be acted on
by a force at right angles to the direction of motion of the particle
and also to the magnetic force, the magnitude of the force being
proportional to the product of the velocity of the particle, the
magnetic force, and the sine of the angle between these vectors. In this
case we have seen that if the particle is not acted on by an
electrostatic field, the path in a uniform magnetic field is a spiral,
which, if the magnetic forc