EINSTEIN ON THE LAW OF
PHOTOCHEMICAL
EQUIVALENCE
I
Einstein's
paper
on
the law
of
photochemical equivalence,
Einstein
1912b
(Doc. 2),
represents
a
continuation of his earlier research
on
the
interaction between
light
and
matter,
and
in
particular
of
his
brief
analysis
of
photochemical processes
in
Einstein
1905i
(Vol. 2,
Doc.
14).[1]
In contrast to this
earlier
work,
the
paper
makes
no use
of
the
light quantum hypothesis
but is
instead based
on an
assumption
about
the rate
of
molecular dissociation and recombination
processes
in
a
substance,
from
which the
photochemical
law is
then derived
by
thermodynamic reasoning.
The
paper
also
pro-
vides
a new
derivation
of
Wien's
formula for
black-body
radiation.
A
supplement
to
the
paper,
Einstein
1912f (Doc. 5),
deals
with the
question
of whether
the
photochem-
ically
active
frequency is
determined
by
the
molecules
interacting
with
radiation,
and
also
with
the
"improper" ("außergewöhnliches") thermodynamic
equilibrium
between
the
radiation and
the
photochemically
active substance.
Both
topics
are
also
discussed
in
Einstein 1913a
(Doc.
12),
the
published
version of
a
lecture Einstein
gave
to the
Societe
francaise
de
physique
on
27
March
1913.
Einstein
1912g (Doc. 6)
is
the
response
to
a
claim
to
priority
by
Johannes Stark
concerning
the
discovery
of
the
law
of
photochemical equivalence (Stark 1912).[2]
II
The
insight
that
not
only heat,
but
also
light
and other kinds
of radiation
may
play
a
role
in
chemical
processes
dates back
to the
nineteenth
century
and
was
viewed
as
evidence for
the
close
relationship
between
the
natural
forces
underlying
chemical
and
physical processes.
In contrast to the
quantitative
study
of heat
in
chemical
pro-
cesses,
which
led to the
development
of
thermochemistry,
a
quantitative
analysis
of
the
effect of
light
on
chemical
processes
remained
problematic
until the first
decade
of
the
twentieth
century.[3]
This
was
due in
part
to
a
lack
of
precision
in
absolute
photometry
and
also to the
difficulty
of
detecting
the small
quantities
of
matter
involved
in
photochemical
reactions.
On the
theoretical
level, photochemical
pro-
cesses
confronted
the
wave theory
of
light
with
a paradox: experiments
indicated that
the
determining
factor
in
photochemical processes
is the
wavelength
of
the
light,
not
its
intensity,
as was
predicted
by
theory.
Einstein summarized
this
difficulty
in
1909:
"Why
does the
occurrence
of
a
certain
photochemical
reaction
only
depend on
the
[1]For
a
historical
study
of Einstein's work
on
photodecomposition, see Bergia
and Navarro
1988.
[2]Einstein also discussed the
law
of
photochemical equivalence
in
the
physics
seminar
he
held
in
winter
semester 1912/1913
and
summer
semester 1913
(see Appendix A).
[3]For
a
contemporary
review,
see Warburg
1917.
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