EINSTEIN ON

PHOTOCHEMICAL EQUIVALENCE

113

Einstein's

view,

the

fact that such

improper equilibria

appeared

to be

impossible

in

the

general case.[25]

Einstein nevertheless

pursued

the

question

in

the

following years

and concluded

his

search with

his

groundbreaking

study

of Planck's law based

on

the

concepts

of

spontaneous

and induced

emission.[26]

The

final

section of Einstein 1916b

is

devoted

to

the law of

photochemical equivalence

and

shows how

its

earlier

deri-

vation conforms

to

the

new

analysis

of Planck's

law.[27]

The

law

of

photochemical equivalence

also

remained

at

the

center

of

Warburg's

interests well after

1912,

in

particular

because

its

experimental

confirmation turned

out to

be

more

difficult

than

he

had

originally expected.

In 1914

Warburg

claimed

that he had obtained results in

close

agreement

with Einstein's

law

for

special

values

of

wavelength

and

pressure,

but

also

found substantial

disagreement

for

higher

wave-

lengths

and

an

unexpected dependence

on

the

pressure.[28]

It

was only

after decades

of

experimentation

that

a

textbook

on

physical chemistry

could

state:

"It

is

now gen-

erally agreed

that the

law

of

the

photochemical

equivalent is always applicable

to

the

primary light-absorbing process,

and

that deviations

are

due to

secondary

processes.

"[29]

[25]See

Einstein

to

Paul

Ehrenfest, 25

April

1912

(Vol. 5,

Doc.

384).

[26]See Einstein

1914j, 1916b,

1916c.

[27]See

Einstein

1916b,

§3.

[28]See

Warburg

1914,

p.

882.

[29]Glasstone

1946,

p.

1163.