12 DIFFERENCE IN POTENTIALS

Doc.

2

ON

THE THERMODYNAMIC THEORY

OF

THE

DIFFERENCE IN POTENTIALS

BETWEEN METALS

AND FULLY

DISSOCIATED SOLUTIONS OF

THEIR

SALTS

AND

ON

AN

ELECTRICAL

METHOD FOR

INVESTIGATING

MOLECULAR FORCES

By

A.

Einstein

[Annalen

der

Physik 8 (1902):

798-814]

§1.

A

hypothetical

extension

of

the second

law of

the

mechanical theory of

heat

The

second

law

of the

mechanical theory of

heat

can

be

applied

to

such

physical

systems

which

are

capable of

passing,

with

any

desired

approximation,

through

reversible

cyclic

processes.

In

accordance with the derivation

of

this

law

from

the

impossibility of

converting

latent heat into mechanical

energy,

it is here

necessary

to

assume

that those

processes

are

realizable.

However,

in

an

important

application of

the mechanical

theory

of

heat,

namely

the

mixing

of

two

or more

gases

by

means

of

semipermeable

membranes,

it is

doubtful whether this postulate is satisfied.

The thermodynamic

theory of

dissociation

of

gases

and

the

theory

of dilute solutions

are

based

on

the

[1]

assumption

that

this

process

is realizable.

As

is well

known,

the

assumption to

be

introduced is

as

follows:

For

any

two

gases

A

and

B

it should

be

possible

to

produce

two

partitions

such

that

one

is

permeable

for

A

but

not

for

B,

while

the other

is

permeable

for

B

but

not

for

A.

If the mixture consists

of

more

than

two

components,

then

this

assumption becomes

even

more

complicated and improbable.

Since the

results of the

theory have been completely confirmed

by

experiment

despite the

fact that

we

worked

with

processes whose

realizability could

indeed

be

doubted,

the

question

arises

whether

the

second

law

could

not be

applied

to

ideal

processes

of

a

certain kind without

contradicting

experience.

In this

sense, on

the basis

of

the

experience

obtained,

we

certainly

can

advance

the proposition:

One

remains

in

agreement

with

experience

if

one

extends the

second

law to physical

mixtures

whose

individual

components

are

restricted

to

certain

subspaces

by

conservative forces

acting

in certain

planes.

We

shall

hypothetically generalize

this

proposition

to

the

following: