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.