INTRODUCTION

TO VOLUME

2

xix

contemporary correspondence

demonstrate

a

familiarity

with

physical

chemistry,

in

particular

the

theory

of

solutions,

as

well

as

a concern

for

finding

experimental

evidence

to

support

his

hypotheses.

At the end

of

1901,

Einstein

attempted unsuc-

cessfully

to

obtain

a

doctorate from

the

University

of

Zurich

with

a

thesis

on

inter-

molecular

forces.[18]

Einstein's

interest

in

a

wide

variety

of

problems involving

the atomic

constitu-

tion

of

matter and

electricity

led him

to

study

the

relationship

between

thermody-

namics and the microstructure

of

various

systems.[19]

In his work

on

inter-

molecular forces in

liquids,

for

example,

he

made extensive

use

of

thermodynamical

methods,

and

even postulated an

extension

of

the

range

of

va-

lidity

of

the second law

of

thermodynamics.

Einstein's

next

three

papers,

pub-

lished between 1902 and

1904,

are on

the foundations of statistical

physics;

their

aim is to establish the

principal thermodynamic properties

that

any macroscopic

system

must have,

with

as

few

assumptions

as

possible

about the constituent ele-

ments

of

the

system,

or

the

nature

of

their interactions.

Presumably as a consequence

of

his

papers

for

the

Annalen

der

Physik

on

topics

in thermal

physics,

Einstein started

to

contribute to the

Beiblätter

zu

den Annalen

der

Physik, reviewing papers

in the

category

"Theory

of

Heat."[20] His services

as a

reviewer

did not last

long,

however;

all but

two

of

his contributions

appeared

in

1905.

In his last

paper on

statistical

physics,

Einstein showed that fluctuations around

the

equilibrium

values

of

thermodynamical quantities

must

occur

as

a conse-

quence

of

the

kinetic

theory

of

heat. He derived

an expression

for the

mean

square

fluctuations

in

the

energy

of

a

system represented by

a

canonical ensemble. The

expression,

which

depends only

on

the

system's

thermodynamical

state,

enabled

him to

interpret

Boltzmann's

constant

as

fixing

the scale for the thermal

stability

of

a

system.

Einstein's

formulae

for

fluctuations became

an

important

tool in his

later research. In addition

to

the

thermodynamic approach,

based

on

equilibrium

statistical

mechanics,

he elaborated

a

second,

distinct

approach

to

fluctuation

phenomena:

the stochastic

approach,

based

on

the

analysis

of

fluctuation-dissipa-

tion mechanisms.

Einstein first

developed

the

latter

approach

in

1905,

when he realized

that

the

motions

of

microscopic particles suspended

in

a

liquid

must

manifest visible fluc-

tuations

owing

to

the molecular structure of the fluid. The methods Einstein used

[18]

For

a

discussion

of Einstein's

first

attempt

to obtain

a

doctorate,

see

the editorial note,

"Einstein's

Dissertation

on

the Determination

of

Molecular Dimensions," section

III,

pp.

173-176.

[19]

See the editorial

note,

"Einstein

on

the

Foundations of Statistical

Physics,"

pp.

45-

46.

[20]

See the editorial

note,

"Einstein's

Re-

views

for

the

Beiblätter

zu

den Annalen

der

Phy-

sik,"

pp.

109-111.