112

EINSTEIN ON

PHOTOCHEMICAL EQUIVALENCE

cided whether

a

particular

effect had

to be

explained

by a

quantum property

of

radi-

ation

or

of

matter.

Einstein's

explanation

of

the

photoelectric

effect

by a quantum

property

of

radiation,

for

instance,

was

challenged

by

Arnold Sommerfeld's claim that

resonance

phenomena

within

the atom

play a

role.[18]

A

similar alternative

presented

itself

in the

case

of

photochemical

reactions, and

Einstein's

original

treatment

did

not

exclude

the

possibility

that

it

was

the

characteristic

frequency

of the

absorbing

mol-

ecules rather than

the

frequency

of

the

incoming

radiation that

matters.[19] He filled

this

gap

with

a

supplement

to his

paper,

Einstein

1912f (Doc. 5),

in which

he

analyzed

the

case

that

a

molecule

can

react to

a

finite

range

of

incoming

frequencies.[20]

An

additional stimulus for

the

theoretical

study

of this

problem

was

provided

by

War-

burg's

recent

experimental

research

on

ozone,

which

he

found

to

display

such

a

finite

range

of

sensitivity.[21]

In his

paper

Einstein assumed that

the

total number of decom-

posed

molecules

is equal

to the

sum

of

those

decomposed

by

the

individual

frequencies

in

order

to

show

that

the

energy

absorbed

by a

molecular dissociation

process depends

only

on

the

frequency

of the radiation. Einstein

presented

his

results

and

carefully

expounded

the

various

assumptions

on

which

they

rest in

a

lecture

he

gave

to

the

Societe

francaise

de

physique on

27

March

1913.[22]

IV

One of

the

key

problems

left unresolved

by

Einstein's

treatment

was

the

question

of

whether Planck's radiation

law

could

be

related

to

photochemical

processes.[23]

Paul

Ehrenfest and Arthur Schidlof

explored

this

possibility independently

and

corre-

sponded

about their

ideas

with

Einstein.[24]

The derivation of Wien's radiation formula

in

Einstein 1912b

(Doc. 2) requires

the

existence of

an "improper" thermodynamic

equilibrium

in

which

the

temperature

of

the

substance

and the

temperature correspond-

ing

to the

incoming

radiation

density

are

different.

The

main

problem

for

extending

this derivation

beyond

the

Wien

range

to

a new

derivation of Planck's

law

was,

in

[18]For

a

discussion of

the

relationship

between

this

problem

and

Einstein's

study

of

pho-

tochemistry,

see

Einstein

to

Wilhelm

Wien,

17

May 1912

(Vol. 5,

Doc.

395).

For Einstein's

controversy

with

Sommerfeld,

see

also

Sommerfeld et

al. 1911

(Vol. 3,

Doc.

24),

and,

for

historical

comment,

Stuewer

1975,

pp.

58-60.

[19]See, e.g.,

Haber's

detailed remarks

in

Fritz Haber

to Einstein,

8

March

1912

(Vol.

5,

Doc.

368),

which

may

have

drawn Einstein's attention

to this

ambiguity.

[20]The

essential

idea of

the

paper

is

sketched

in

Einstein

to

Emil

Warburg,

after

25

April–

before

11

May 1912

(Vol.

5,

Doc.

386).

[21]For

evidence

that

Warburg

communicated his

findings

to Einstein,

see

Einstein

1912f

(Doc. 5),

p.

881.

See

also Einstein

to

Emil

Warburg,

25

April

1912

(Vol. 5,

Doc.

385).

[22]See

Einstein

1913a

(Doc.

12)

for

its

published

version.

[23]For

Einstein's

early

interest

in this

question, see

Einstein

to

Michele

Besso, 26

March

1912

(Vol. 5,

Doc.

377).

[24]See Paul

Ehrenfest

to Einstein,

before

3 April

1912

(Vol. 5,

Doc.

380);

Einstein

to

Arthur

Schidlof,

17

June

1913

(Vol. 5,

Doc. 446);

and Einstein

to

Arthur

Schidlof,

5

July 1913

(Vol.

5,

Doc. 449).