NATURE OF

MOLECULAR FORCES

5

presumed

analogy

with

gravitational

forces,

he

postulated a

similar form for the

potential

between two molecules:

P

=

P*-

C1C2p(r),

where

p(r)

is

a

universal function

of

the distance

r

between the molecules. At the

time,

such

an expression

for the intermolecular

potential was

not uncommon;

the

masses

of

the

interacting

molecules

were usually

chosen

as

the constants

c.[12]

In

Einstein's

paper,

how-

ever,

the constants

c1

and

c2

are

assumed to

depend on

the chemical nature

of

the

mole-

cules. The atoms

of

a

chemical element

are

characterized

by

a

certain value

of

c;

the

c

of

a

molecule is assumed to

be the

sum

of

the

c's of

its constituent atoms.

Einstein did

not

discuss the

range

of

the intermolecular

force in the

paper,

but

his

assumptions

about the

form

of

the

potential imply

that the

range

is

the

same

for all molecules.[13] Einstein derived

relations between the constants

c

for the molecules

of

a

neutral

liquid

and several

measur-

able

properties

of

the

liquid. Comparison

with

experimental

values

of

these

properties

for

various

liquids

allowed him to establish

(relative)

values

of

c

for

a

number

of

elements.[14]

From these,

he

calculated the

values for

a

number

of

compounds,

which he then

compared

with the

experimentally

established values for the latter. On the

whole,

he found the

agree-

ment between observed and calculated values to be

satisfactory.

Einstein continued to

work

on

the

topic

of

liquid

surfaces

during

1901,

apparently

without

any

further

suc-

cess.[15]

He also derived

a consequence

from his

theory

that

appeared

to be

in

contradiction

with the Van der Waals

theory

of

liquids.[16]

After the

paper was published on

1

March

1901,

offprints were

sent to several

promi-

nent

physicists,

notably

Wilhelm Ostwald

(who

is

cited

in

the

paper), as part

of Einstein's

unsuccessful

attempt during

that

spring

to

find

a

position as

Assistent

at

some

univer-

sity.[17]

In

a

letter

to

Ostwald

accompanying

the

offprint,

Einstein

expressed

his indebt-

edness to

Ostwald's

work.[18]

If

Ostwald read the

paper,

he

was presumably

not

sympa-

thetic to

Einstein's

molecular

approach.

Ostwald

had

been

an

ardent

energeticist

for

some

time,

and

was quite

hostile to atomistic

explanations.[19]

In

1891

he

already

wrote

some-

[12] See, e.g.,

Boltzmann 1898a,

p.

56.

[13] A

decade later he noted that this

assump-

tion

is not

tenable

(see

Einstein

1911a,

and the

discussion

of

it

below).

[14]

One

of

the

constants

he calculated turned

out to

be

negative, implying

the

possibility

of

a

repulsive

force between molecules. Einstein did

not

comment

on

this

question.

[15]

See Einstein to Mileva

Maric,

22

July

1901 (Vol.

1,

Doc.

119).

[16]

See Einstein

to

Mileva

Maric, 12

Decem-

ber

1901 (Vol.

1,

Doc.

127).

The

problem arose

from

Einstein's

tacit

assumption

that the

range

of

molecular forces

is

the

same

for all mole-

cules.

[17]

See Einstein to Otto

Wiener, 9

March

1901 (Vol.

1,

Doc.

90);

Einstein to Wilhelm

Ostwald,

19

March

1901 (Vol.

1,

Doc. 92); and

Einstein

to

Heike

Kamerlingh Onnes,

12

April

1901 (Vol.

1,

Doc.

98).

In his letter

to Wiener,

Einstein

only

mentions

his

paper, presumably

because he had

not

yet

received

offprints.

[18]

Einstein

to

Wilhelm

Ostwald, 19

March

1901 (Vol.

1,

Doc.

92).

Einstein

states

that read-

ing

Ostwald

1891 had stimulated Einstein 1901

(Doc.

1);

but he

seems

to

have used

Ostwald's

book

more as a source

of

data

on capillary phe-

nomena

than

as a source

of

theoretical ideas

on

molecular

forces.

[19]

For

a

discussion

of

the

development

of

Ostwald's

views

on energetics, see

Deltete

1983,

chap.

V

and

appendix

I.