101

out the lines of the alkali; for example LI CI give out the red
lithium line and sodium salts the D line. It is remarkable
the lines doe to the metal are more easily excited in the
than In the metal themselves. Thus if the liquid alloy of sodium
and potassium is bombarded by positive rays the specks of
oxide on the surface glow brightly with the sodium light
while the clean surface remains quite dark. Some observers
have noticed what seems a similar effect with hydrogen, viz.
that the hydrogen lines are more easily excited in water
vapour than in pure hydrogen. The fact that in the positive
ray photographs, the parabolas corresponding to a certain
type of ray^ for example the carbon or oxygen atom with two
charges, is more easily developed from compounds than from
the molecules of the gases themselves is probably connected
with this effect

The production of spectra by bombardment with cathode
rays has been investigated by Gyllenskold (" Ark. £ Math. Ast.
oet Fys./J 4, No. 33, 1908), and by Stark and Wendt ("Ann.
der Phys.," 88, p. 669, 1912) who have shown that the colour-
less salts of the alkalies and alkaline earths and also of thallium,
zinc, and aluminium give out the series lines of the metal when
struck by the positive rays and that the lines given out do not
depend upon the character of the salts. According to Stark
and Wendt the seat of the emission is not the surface of the salt
itself but a layer of gas, less than I mm. thick, close to the
surface. This layer is analogous to the velvety glow which
covers the surface of the cathode where an electric discharge
passes through a gas at a low pressure.

To develop the spectrum of the metal the positive rays
must have more than a certain critical amount of energy
depending on the nature of the salt. The values of V, this
critical energy, measured by the number of volts through
which the atomic charge must fall to acquire it, have beenrecombinations