EINSTEIN ON

PHOTOCHEMICAL

EQUIVALENCE

111

Beginning

in

1907,

Emil

Warburg published

a

series

of

papers

in

which

he

sought

theoretically

to

establish

the

energy

balance of

photochemical

reactions.[11] He

attempted to

extend thermochemical methods

to these

reactions

by

relying

on

the

results of Wilhelm Wien

and

Max Planck

on

the

thermodynamics

of

radiation,

and

became convinced that

his

results

implied

the

existence of

a

sensitivity

threshold.[12]

By

1911

Warburg

had

also

begun

an

experimental study

of

the

photochemical energy

balance.[13]

Einstein

and

Warburg

met at the first

Solvay

Congress

in

1911,

and

in

an

exchange

of views

on

photochemistry

Einstein

disputed Warburg's

earlier results

and

formulated

his

thermodynamic proof

of

the law

of

photochemical

equivalence.[14]

Warburg

then

set out to

undertake

its

experimental

verification.[15]

III

The

law

of

photochemical equivalence

states

that

the

decomposition

of

a

gram-equiv-

alent of

a

substance

by

radiation of

frequency

v

requires

an

energy

E

=

Nhv,

where

N

is

Avogadro's

number and

h

is

Planck's

constant. The

name

of

the law

recalls

Faraday's

law

of

electrochemical

equivalence.[16]

The derivation of

this law in

Einstein

1912b

(Doc. 2)

is

based

on

the

assumption

that the chemical effect of

radiation

depends

only

on

its

energy;

the law is

limited

to

radiation of

low

density

within

the

range

of

validity

of Wien's radiation

law. For

radiation within

this

range,

Wien's formula

is

also

a

consequence

of Einstein's

assumptions.

Einstein's

thermodynamic

derivation of results that

are

closely

related

to the

quan-

tum

hypothesis

made

it

possible to

reexamine

some

of

the issues

that

this

hypothesis

had

raised

in

contemporary

discussions.[17] In

these debates

it often

remained unde-

[11]See

Warburg 1907,

1909.

[12]See Warburg 1907,

p.

755,

and

Einstein

to

Heinrich

Zangger,

20

November

1911

(Vol.

5,

Doc.

309).

[13]See

Warburg

1911.

[14]For evidence,

see

Einstein

to

Heinrich

Zangger,

20

November

1911 (Vol.

5,

Doc.

309);

Einstein

to

Michele

Besso,

4

February 1912

(Vol. 5,

Doc.

354);

and

Warburg 1912,

p.

224.

Einstein

also

mentioned

photochemical

reactions in

his

Solvay

lecture

(see

Einstein 1914a

[Vol.

3,

Doc.

26], p. 348).

[15]On

3 February

1912

Einstein received

a

letter from

Warburg concerning

the

experimental

confirmation of

the

equivalence

law

(see

Einstein

to

Michele

Besso,

4

February

1912

[Vol.

5,

Doc.

354]);

for

the publication

of

Warburg's results,

see

Warburg 1912.

For evidence that

Einstein had stimulated

Warburg's research,

see

also

Einstein

to

Heinrich

Zangger,

before

29

February

1912

(Vol.

5,

Doc.

366).

[16]See,

e.g.,

Fritz Haber

to Einstein,

8

March

1912

(Vol.

5,

Doc.

368).

[17]See Einstein

to

Wilhelm

Wien,

11 May

1912

(Vol.

5,

Doc.

392),

and

Einstein

to Wilhelm

Wien,

17

May

1912

(Vol. 5,

Doc.

395).

Other letters

in

which

the

relationship

between

the law

of

photochemical equivalence

and

the

quantum theory

is

mentioned

are

Einstein

to

Heinrich

Zangger,

27

January

1912

(Vol. 5,

Doc.

344);

Einstein

to

Michele

Besso, 4

February 1912

(Vol.

5,

Doc.

354);

Einstein

to H. A. Lorentz,

18 February

1912

(Vol.

5,

Doc.

360) (this

letter

contains

a

detailed

account

of

the

derivation of

the

law);

and

Einstein

to Ludwig

Hopf,

after

20

February

1912

(Vol. 5,

Doc.

364).