284
EINSTEIN ON
CRITICAL OPALESCENCE
dynamics.[8]
Einstein later characterized his achievement
as
a
"quantitative
realiza-
tion of the
theory by Smoluchowski."[9]
Einstein's
and
Smoluchowski's lines
of research had crossed
on an
earlier
occasion,
on
their
work
on
Brownian motion
in 1905
and
1906.[10]
In the
case
of
critical
opales-
cence
their
common
interest
in
the atomistic constitution of
matter
once
again
led
them
on
similar
paths,
which each followed
by
developing
his
characteristic
style
of
doing
physics.[11]
Einstein's
first encounter
with critical
opalescence
dates from
his
student
days;
so
his
interest
in
the
subject may already
have been aroused
by
the time
he
read Smoluchowski's
paper.
H. F.
Weber's
physics course,
which Einstein took
between
1897
and
1898
as a
student
at
the ETH
in Zurich,
touched
briefly
upon
critical
phenomena
and the
history
of their
discovery,
including
the
discovery
of
opalescence.[12]
Einstein's
paper
on
critical
opalescence
follows
several of
his
earlier
papers
in
presenting yet
another method for
determining Avogadro's
number.[13]
His
continued effort
to find
new
ways
of
determining
this number
was
motivated, among
other
reasons,
by
the role
it
played
in
the discussions
about
Planck's formula for
black-body
radiation
and
its
problematic
status in
contemporary
physics.[14]
But
Einstein's work
on
critical
opalescence
also
contributes to
the
realization
of
a
more
general goal
that
had
guided
his
earlier
research
on
statistical
physics,
the
goal
of
establishing
the molecular constitution
of
matter
as firmly as
possible.
Einstein's
key
insight
in
his paper
was
that the
phenomena
of critical
opalescence
and the blue color of
the
sky,
which
are
not
obviously
related
to each
other,
are
both
due
to density
fluctuations caused
by
the molecular constitution of
matter.[15]
Even
[8]For Einstein's critical
views
on
Keesom's
contribution,
see
Einstein
to W. H.
Julius,
18
December
1911,
also
on
the
fact
that
he
did
not
include
a positive
remark
on
Keesom,
which
he
had drafted
in his
published
discussion remarks to the
Solvay Congress
(for
Einstein's
draft,
see
Doc.
25,
p. 509).
For evidence of Einstein's earlier
positive
evaluation of Keesom's contribu-
tion, see
also Heike
Kamerlingh
Onnes
to
W. H.
Julius,
24 November
1911,
NeUU,
Archief
Julius
I,
27a.
[9]"Quantitative
Durchführung
der Theorie
von
Smoluchowski."
See
Einstein
to Jakob
Laub, 27
August 1910;
for evidence of Einstein's enthusiasm
concerning
his
work
on
critical
opalescence,
see
also Einstein
to
Jakob
Laub,
11
October
1910.
[10]For
an
overview of Einstein's and Smoluchowski's work
on
Brownian
motion,
see
Vol.
2,
the editorial
note,
"Einstein
on
Brownian
Motion," pp.
206-222.
[11]For
a
comparative study
of Einstein's and Smoluchowski's
work,
see
Teske
1969. Einstein
reviewed Smoluchowski's
approach
to
providing
evidence for
atomism
in
his
obituary
of
Smoluchowski,
Einstein
1917.
[12]For Weber's
treatment
of the
history
of the
discovery
of critical
phenomena,
see
"H.
F.
Weber's Lectures
on
Physics,"
ca.
December 1897-June
1898
(Vol.
1,
Doc.
37),
pp.
145-146. In
the
margin
of
his
notes
on
Weber's
explanation
of
a
phase transition,
Einstein
wrote
"obscure
point" ("Dunkler Punkt").
[13]This point
is
emphasized
and elaborated
upon
in Pais
1982,
pp.
100-104.
[14]See
Einstein
to
Jean
Perrin,
11
November
1909,
where Einstein
points
out
this motiva-
tion.
For the
historical
context, see
Vol.
2,
the editorial
note,
"Einstein's
Early
Work
on
the
Quantum Hypothesis," pp.
134-148.
[15]For
a
discussion
of the different
views
of Einstein
and Smoluchowski
on
this
issue,
see
Teske 1969.