EINSTEIN ON
PHOTOCHEMICAL EQUIVALENCE
113
Einstein's
view,
the
fact that such
improper equilibria
appeared
to be
impossible
in
the
general case.[25]
Einstein nevertheless
pursued
the
question
in
the
following years
and concluded
his
search with
his
groundbreaking
study
of Planck's law based
on
the
concepts
of
spontaneous
and induced
emission.[26]
The
final
section of Einstein 1916b
is
devoted
to
the law of
photochemical equivalence
and
shows how
its
earlier
deri-
vation conforms
to
the
new
analysis
of Planck's
law.[27]
The
law
of
photochemical equivalence
also
remained
at
the
center
of
Warburg's
interests well after
1912,
in
particular
because
its
experimental
confirmation turned
out to
be
more
difficult
than
he
had
originally expected.
In 1914
Warburg
claimed
that he had obtained results in
close
agreement
with Einstein's
law
for
special
values
of
wavelength
and
pressure,
but
also
found substantial
disagreement
for
higher
wave-
lengths
and
an
unexpected dependence
on
the
pressure.[28]
It
was only
after decades
of
experimentation
that
a
textbook
on
physical chemistry
could
state:
"It
is
now gen-
erally agreed
that the
law
of
the
photochemical
equivalent is always applicable
to
the
primary light-absorbing process,
and
that deviations
are
due to
secondary
processes.
"[29]
[25]See
Einstein
to
Paul
Ehrenfest, 25
April
1912
(Vol. 5,
Doc.
384).
[26]See Einstein
1914j, 1916b,
1916c.
[27]See
Einstein
1916b,
§3.
[28]See
Warburg
1914,
p.
882.
[29]Glasstone
1946,
p.
1163.
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