BROWNIAN MOTION 211

nehmbare

ungeordnete Bewegung

ausführen müssen, welche durch die Wär-

mebewegung

erzeugt ist.[35]

Einstein wrote this

paper

"without

knowing

that observations

concerning

Brownian

motion

were already long

familiar"

("ohne

zu

wissen,

dass

Beobachtungen

über die

'Brown'sche

Bewegung'

schon

lange

bekannt

waren").[36]

He did not mention Brownian

motion

in the

title

of

Einstein 1905k

(Doc. 16),

although

in the text he

conjectured

that

the

motion he

predicted

might

be identical to Brownian

motion.[37]

Boltzmann's

Gastheorie,

which Einstein

carefully

studied

during

his student

years, explicitly

denies

that

the thermal

motion

of

molecules

in

a

gas

leads to observable motions

of

suspended

bodies.[38] (This

denial

may

be

an

instance

of

what

Einstein referred

to

as

Boltzmann's

attaching

too little

importance

to

a

comparison

of

theory

with

observation.[39])

Some time between

1902 and

1905,

Einstein read

Poincare's

Science

et hypothese,

which

contains

a

brief

discussion

of

Gouy's

work

on

Brownian

motion,

emphasizing

Gouy's

argument

that Brownian motion

violates the second law

of

thermodynamics.[40]

Einstein's

second

paper

on

Brownian

mo-

tion,

written after

Siedentopf

drew his

attention to

Gouy's

work,[41]

cites the

observations

reported

in

Gouy

1888

as qualitative

confirmation

of

his results.

Einstein

1905k

(Doc.

16)

opens

with the derivation

of

an expression

for the coefficient

of

diffusion

in

terms

of

the radius

of

the

suspended particles,

and the

temperature

and

viscosity

of

the

liquid, an expression already

obtained in

Einstein's

doctoral disserta-

tion.[42]

Unlike the

previous

derivation,

however,

the

new one

makes

use

of

the methods

of

statistical

physics

that Einstein

developed.

The

new

approach

is

different

in two

respects:

(1)

In his dissertation, which deals with solutions rather than

suspensions,

Einstein

simply

assumed the

validity

of

Van

't Hoff's

law for the osmotic

pres-

sure.

He

now gave a

derivation

of

this

law from

an expression

for the free

energy

of

the

suspension

that follows from statistical mechanics.

(2)

Rather than

simply considering

the

equilibrium

of

forces

acting on a single

molecule,

Einstein derived the

equilibrium

between the osmotic

pressure

and

a

friction force

obeying

Stokes's law

by

a

thermodynamical argument.[43]

[35]

Einstein to Conrad

Habicht, 18

May-8

June

1905.

[36]

Einstein

1979,

p.

44; translation,

p.

45.

See also Einstein

to

Michele

Besso,

6

January

1948,

and Einstein to Carl

Seelig,

15

September

1952.

[37]

In Einstein

to

Conrad

Habicht, 18 May-8

June

1905,

he wrote:

"Unexplained move-

ments

of

inanimate small

suspended

bodies have

in

fact

been observed

by

physiologists, move-

ments which

they

call

'Brownian

molecular

mo-

tion'

" ("[E]s

sind

unerklärte

Bewegungen

lebloser kleiner

suspendirter

Körper

in der That

beobachtet

worden

von

den

Physiologen,

welche

Bewegungen

von

ihnen

'Brown-sche

Molekularbewegung'

genannt

wird").

[38]

See Boltzmann

1898a,

pp.

111-112.

For

evidence

that Einstein read the Gastheorie,

see

note 12.

[39]

See

note 15

above.

[40]

See

Poincare

1902,

p.

209. For

Einstein's

reading

of

Poincare,

see

the Introduction,

p.

xxv.

[41]

See

Einstein

1906b

(Doc. 32), p.

371.

[42]

See

Einstein

1905j (Doc. 15), p.

19.

[43]

A sketch

of

this

derivation is found

in

Ein-

stein

1905j (Doc. 15),

§

4. Some

of Einstein's

contemporaries

considered the

application

of