ill
be less than that of a particle which has retained its positive charge
for the whole of its journey, while the few particles which have a
negative charge for a longer time than they have a positive will be
deflected in the opposite direction to the main portion and will produce
the tail (fig. 27).

[Illustration: Fig. 27.]

A similar explanation will apply to the positive rays discovered by
Villard (_Comptes rendus_, 143, p. 674) and J. J. Thomson (_Phil. Mag._
13, p. 359), which travel in the opposite direction to the rays we have
been considering, i.e. they travel away from the cathode and in the
direction of the cathode's rays; these rays are sometimes called
"retrograde" rays. These as far as has been observed have always the
same maximum value of e/m, i.e. 10^4, and there are a considerable
number of negative ones always mixed with them. The maximum velocity of
both the positive and retrograde rays is about 2 X 10^8 cm./sec. and
varies very little with the potential difference between the electrodes
in the tube in which they are produced (J. J. Thomson, _Phil. Mag._,
Dec. 1909).

The positive rays show, when the pressure is not very low, the line
spectrum of the gas through which they pass. An exceedingly valuable set
of observations on this point have been made by Stark and his pupils
(_Physik. Zeit._ 6, p. 892; _Ann. der Phys._ 21, pp. 40, 457). Stark has
shown that in many gases, notably hydrogen, the spectrum shows the
Doppler effect, and he has been able to calculate in this way the
velocity of the positive rays.

_Anode Rays._--Gehrcke and Reichenhein (_Ann. der Phys._ 25, p. 861)
have found that when the anode consists of a mixture of sodium and
lithium chloride raised to a high temperature either by the discharge
itself or by an independent heating circuit, very conspicuous rays come
from the anode when the pressure of the gas in the discharge tube is
very low, and a large coil is used to produce the discharge. The
determination of e/m for these rays showed that they are positively
charged atoms of sodium or lithium, moving with very considerable
velocity; in some of Gehrcke's experiments the maximum velocity was as
great as 1.8 X 10^7 cm./sec. though the average was about 10^7 cm./sec.
These velocities are less than those of the positive rays whose maximum
velocity is about 2 X 10^8 cm./sec. (J. J. T.)

FOOTNOTES:

[1] The values for nickel and bismuth given in the table are much
h