EINSTEIN ON
PHOTOCHEMICAL
EQUIVALENCE
111
Beginning
in
1907,
Emil
Warburg published
a
series
of
papers
in
which
he
sought
theoretically
to
establish
the
energy
balance of
photochemical
reactions.[11] He
attempted to
extend thermochemical methods
to these
reactions
by
relying
on
the
results of Wilhelm Wien
and
Max Planck
on
the
thermodynamics
of
radiation,
and
became convinced that
his
results
implied
the
existence of
a
sensitivity
threshold.[12]
By
1911
Warburg
had
also
begun
an
experimental study
of
the
photochemical energy
balance.[13]
Einstein
and
Warburg
met at the first
Solvay
Congress
in
1911,
and
in
an
exchange
of views
on
photochemistry
Einstein
disputed Warburg's
earlier results
and
formulated
his
thermodynamic proof
of
the law
of
photochemical
equivalence.[14]
Warburg
then
set out to
undertake
its
experimental
verification.[15]
III
The
law
of
photochemical equivalence
states
that
the
decomposition
of
a
gram-equiv-
alent of
a
substance
by
radiation of
frequency
v
requires
an
energy
E
=
Nhv,
where
N
is
Avogadro's
number and
h
is
Planck's
constant. The
name
of
the law
recalls
Faraday's
law
of
electrochemical
equivalence.[16]
The derivation of
this law in
Einstein
1912b
(Doc. 2)
is
based
on
the
assumption
that the chemical effect of
radiation
depends
only
on
its
energy;
the law is
limited
to
radiation of
low
density
within
the
range
of
validity
of Wien's radiation
law. For
radiation within
this
range,
Wien's formula
is
also
a
consequence
of Einstein's
assumptions.
Einstein's
thermodynamic
derivation of results that
are
closely
related
to the
quan-
tum
hypothesis
made
it
possible to
reexamine
some
of
the issues
that
this
hypothesis
had
raised
in
contemporary
discussions.[17] In
these debates
it often
remained unde-
[11]See
Warburg 1907,
1909.
[12]See Warburg 1907,
p.
755,
and
Einstein
to
Heinrich
Zangger,
20
November
1911
(Vol.
5,
Doc.
309).
[13]See
Warburg
1911.
[14]For evidence,
see
Einstein
to
Heinrich
Zangger,
20
November
1911 (Vol.
5,
Doc.
309);
Einstein
to
Michele
Besso,
4
February 1912
(Vol. 5,
Doc.
354);
and
Warburg 1912,
p.
224.
Einstein
also
mentioned
photochemical
reactions in
his
Solvay
lecture
(see
Einstein 1914a
[Vol.
3,
Doc.
26], p. 348).
[15]On
3 February
1912
Einstein received
a
letter from
Warburg concerning
the
experimental
confirmation of
the
equivalence
law
(see
Einstein
to
Michele
Besso,
4
February
1912
[Vol.
5,
Doc.
354]);
for
the publication
of
Warburg's results,
see
Warburg 1912.
For evidence that
Einstein had stimulated
Warburg's research,
see
also
Einstein
to
Heinrich
Zangger,
before
29
February
1912
(Vol.
5,
Doc.
366).
[16]See,
e.g.,
Fritz Haber
to Einstein,
8
March
1912
(Vol.
5,
Doc.
368).
[17]See Einstein
to
Wilhelm
Wien,
11 May
1912
(Vol.
5,
Doc.
392),
and
Einstein
to Wilhelm
Wien,
17
May
1912
(Vol. 5,
Doc.
395).
Other letters
in
which
the
relationship
between
the law
of
photochemical equivalence
and
the
quantum theory
is
mentioned
are
Einstein
to
Heinrich
Zangger,
27
January
1912
(Vol. 5,
Doc.
344);
Einstein
to
Michele
Besso, 4
February 1912
(Vol.
5,
Doc.
354);
Einstein
to H. A. Lorentz,
18 February
1912
(Vol.
5,
Doc.
360) (this
letter
contains
a
detailed
account
of
the
derivation of
the
law);
and
Einstein
to Ludwig
Hopf,
after
20
February
1912
(Vol. 5,
Doc.
364).