102 HEURISTIC
VIEW
OF
LIGHT
traverse
in order
to
produce
light that is
just
visible
amounts to
several
hundred
volts
in
some cases,
and
to
thousands
of
volts
in
others.1
We
must
therefore
assume
that the kinetic
energy
of
one
electron
is
used
for the
production
of
many
quanta
of
light
energy.
[48]
§9. On
the
ionization
of gases
by
ultraviolet
light
We
will
have to
assume
that
in
the ionization of
a gas by
ultraviolet
light
one
quantum
of
light
energy
is
used
for the ionization
of
one
molecule
of
gas.
From
this it follows that the
work
of
ionization
(i.e.,
the
work
theoretically
required for ionization)
of
one
molecule cannot be greater
than
the
energy
of
one
effective
quantum
of light
absorbed. If J denotes the
(theoretical)
ionization
work
per
gram-equivalent,
we
must
have
Rßv
I
J
.
However,
according to
measurements
by
Lenard,
the
largest
effective
wavelength
[49]
for air is about 1.9
x
10-5cm,
hence
Rßv
=
6.4
x
1012
erg
£
J
.
An
upper
limit for the
work
of
ionization
can
also
be
obtained
from
the
ionization potentials in rarefied
gases.
According
to
J.
Stark2
the smallest
measured
ionization
potential
(at
platinum anodes)
for air is
about
10
volts.3
Thus
one
obtains 9.6
x
1012
as
the
upper
limit for J,
which
is
almost
equal
to
the value
we
have
just
found. There is
still
another
consequence, whose
verification
by
experiment
seems
to
me
of great importance.
If
each
absorbed
quantum
of
light
energy
ionizes
one
molecule, then the
following
relation
[47] 1P.
Lenard, Ann.
d.
Phys. 12
(1903):
469.
2J.
Stark,
Die
Elektrizität
in
Gasen,
p.
57.
Leipzig, 1902
3In
the interior
of
the
gases
the ionization
potential of
negative
ions is
five times
larger,
however.
[50]