y.

These rays striking against glass make it phosphorescent. The colour of
the phosphorescence depends on the kind of glass; thus the light from soda
glass is a yellowish green, and that from lead glass blue. Many other
bodies phosphoresce when exposed to these rays, and in particular the
phosphorescence of some gems, such as rubies and diamonds, is exceedingly
vivid. The spectrum of the phosphorescent light is generally continuous,
but Crookes showed that the phosphorescence of some of the rare earths,
such as yttrium, gives a spectrum of bright bands, and he founded on this
fact a spectroscopic method of great importance. Goldstein (_Wied. Ann._
54, p. 371) discovered that the haloid salts of the alkali metals change
colour under the rays, sodium chloride, for example, becoming violet. The
coloration is a surface one, and has been traced by E. Wiedemann and
Schmidt (_Wied. Ann._ 54, p. 618) to the formation of a subchloride.
Chlorides of tin, mercury and lead also change colour in the same way. E.
Wiedemann (_Wied. Ann._ 56, p. 201) discovered another remarkable effect,
which he called thermo-luminescence; he found that many bodies after being
exposed to the cathode rays possess for some time the power of becoming
luminous when their temperature is raised to a point far below that at
which they become luminous in the normal state. Substances belonging to
the class called by van 't Hoff solid solutions exhibit this property of
thermo-luminescence to a remarkable extent. They are formed when two
salts, one greatly in excess of the other, are simultaneously precipitated
from a solution. A trace of MnSO4 in CaSO4 shows very brilliant
thermo-luminescence. The impact of cathode rays produces after a time
perceptible changes in the glass. Crookes (_Phil. Trans._ pt. ii. 1879, p.
645) found that after glass has been phosphorescing for some time under
the cathode rays it seems to get tired, and the phosphorescence is not so
bright as it was initially. Thus, for example, when the shadow of a
Maltese cross is thrown on the walls of the tube as in fig. 23, if after
the discharge has been going on for some time the cross is shaken down or
a new cathode used whose line of fire does not cut the cross, the pattern
of the cross will still be seen on the glass, but it will now be brighter
instead of darker than the surrounding portion. The portions shielded by
the cross, not being tired by being made to phosphoresce for a long time,
re