90 DOC.
2
LAW OF PHOTOCHEMICAL
EQUIVALENCE
m1 m2
+
m3
and
m2
+
m3 m1.
occur
with
equal frequency.
We wish
to
consider the
case
in
which the
decomposition
of the molecules
m1
occurs
exclusively
as a
result of the effect of
the
thermal
radiation, namely
due
to
the
effect of
a
part
of the thermal radiation whose
frequency
differs little from
a
specific
[2] frequency v0.
Let the
average
radiation
energy
absorbed in such
a decomposition
be
e.
In this
case,
radiation of the
frequency region
v0,
and
only
radiation of the
frequency region
v0,
must,
conversely,
be emitted
in
the
process
of
combining
m2
and
m3
into
m1,
and the radiation
energy
emitted
in
a
recombination
process
must
also be
on
the
average
e,
because otherwise the existence of the
gas
would disturb
the
equilibrium,
for the number of
decomposition processes
is
equal
to
the number
of
combination
processes.
If
the
gas
mixture
possesses
the
temperature
T,
then
the
system can by
all
means
be in
thermodynamic equilibrium
if the radiation in the
neighborhood
of the
frequency
v0
that is
present
in
the
space possesses
the
(monochromatic) density
p
corresponding
to
thermal radiation of
temperature T.
Let
us now
analyze
more
closely
the
two
exactly
counterbalancing
reactions
by making
some
assumptions
about their
mechanism.
We
assume
that the
decomposition
of
a
molecule of the first kind
proceeds
as
if
the other molecules
were
not
present (assumption I).
From this it
follows
that
we
have
to
postulate
that the number of molecules of the first kind
decomposing per
unit
time
is
proportional
to
their
number
(n1)
under otherwise
equal
circumstances,
and
that the number of molecules
decomposing per
unit time
is
independent
of
the
densities of
the three
gases.
In
addition,
we assume
that the
probability
that
a
molecule
of
the first
kind
decomposes during
a
unit of time is
proportional
to
the
monochromatic radiation
density
p
(assumption II).
It
must
be
emphasized,
as
regards mainly
the second of these
assumptions,
that
its
correctness
is
not at all
self-evident.
It
includes the
proposition
that the chemical
effect of radiation
impinging on a body depends only on
the total
quantity
of the
acting radiation,
but
not
on
the
intensity
of the
radiation;
the existence
of
a
lower
action threshold for the
radiation
is
completely
ruled
out
by
this
assumption.
The
[4]
latter
puts us
in conflict with
the results of
two
papers by
E.
Warburg,3
which
stimulated
me
to
undertake the
present
work.
It follows from the
two
assumptions
that the number Z of the molecules of the
first kind that
decompose per
unit time
is
given by
the
expression
[3]
3E.
Warburg,
Verh.
d.
Deutsch.
Physik.
Ges.
9
(1908):
24 and
9
(1909):
21.