cles are moving
with a velocity comparable with that of light their masses increase
rapidly with their velocity. This effect has been detected by Kauffmann
(_Gott. Nach._, Nov. 8, 1901), who used the corpuscles shot out from
radium, some of which move with velocities only a few per cent less than
that of light. Other experiments on this point have been made by
Bucherer (_Ann. der Phys._ 28, p. 513).

_Conductivity Produced by Ultra-Violet Light._--So much use has been
made in recent times of ultra-violet light for producing ions that it is
desirable to give some account of the electrical effects produced by
light. The discovery by Hertz (_Wied. Ann._ 31, p. 983) in 1887, that
the incidence of ultra-violet light on a spark gap facilitates the
passage of a spark, led to a series of investigations by Hallwachs,
Hoor, Righi and Stoletow, on the effect of ultra-violet light on
electrified bodies. These researches have shown that a freshly cleaned
metal surface, charged with negative electricity, rapidly loses its
charge, however small, when exposed to ultra-violet light, and that if
the surface is insulated and without charge initially, it acquires a
positive charge under the influence of the light. The magnitude of this
positive charge may be very much increased by directing a blast of air
on the plate. This, as Zeleny (_Phil. Mag._ [5], 45, p. 272) showed, has
the effect of blowing from the neighbourhood of the plate negatively
electrified gas, which has similar properties to the charged gas
obtained by the separation of ions from a gas exposed to Rontgen rays or
uranium radiation. If the metal plate is positively electrified, there
is no loss of electrification caused by ultra-violet light. This has
been questioned, but a very careful examination of the question by
Elster and Geitel (_Wied. Ann._ 57, p. 24) has shown that the apparent
exceptions are due to the accidental exposure to reflected ultra-violet
light of metal surfaces in the neighbourhood of the plate negatively
electrified by induction, so that the apparent loss of charge is due to
negative electricity coming up to the plate, and not to positive
electricity going away from it. The ultra-violet light may be obtained
from an arc-lamp, the effectiveness of which is increased if one of the
terminals is made of zinc or aluminium, the light from these substances
being very rich in ultra-violet rays; it may also be got very
conveniently by sparking with an induction