90 DOC.

2

LAW OF PHOTOCHEMICAL

EQUIVALENCE

m1 m2

+

m3

and

m2

+

m3 m1.

occur

with

equal frequency.

We wish

to

consider the

case

in

which the

decomposition

of the molecules

m1

occurs

exclusively

as a

result of the effect of

the

thermal

radiation, namely

due

to

the

effect of

a

part

of the thermal radiation whose

frequency

differs little from

a

specific

[2] frequency v0.

Let the

average

radiation

energy

absorbed in such

a decomposition

be

e.

In this

case,

radiation of the

frequency region

v0,

and

only

radiation of the

frequency region

v0,

must,

conversely,

be emitted

in

the

process

of

combining

m2

and

m3

into

m1,

and the radiation

energy

emitted

in

a

recombination

process

must

also be

on

the

average

e,

because otherwise the existence of the

gas

would disturb

the

equilibrium,

for the number of

decomposition processes

is

equal

to

the number

of

combination

processes.

If

the

gas

mixture

possesses

the

temperature

T,

then

the

system can by

all

means

be in

thermodynamic equilibrium

if the radiation in the

neighborhood

of the

frequency

v0

that is

present

in

the

space possesses

the

(monochromatic) density

p

corresponding

to

thermal radiation of

temperature T.

Let

us now

analyze

more

closely

the

two

exactly

counterbalancing

reactions

by making

some

assumptions

about their

mechanism.

We

assume

that the

decomposition

of

a

molecule of the first kind

proceeds

as

if

the other molecules

were

not

present (assumption I).

From this it

follows

that

we

have

to

postulate

that the number of molecules of the first kind

decomposing per

unit

time

is

proportional

to

their

number

(n1)

under otherwise

equal

circumstances,

and

that the number of molecules

decomposing per

unit time

is

independent

of

the

densities of

the three

gases.

In

addition,

we assume

that the

probability

that

a

molecule

of

the first

kind

decomposes during

a

unit of time is

proportional

to

the

monochromatic radiation

density

p

(assumption II).

It

must

be

emphasized,

as

regards mainly

the second of these

assumptions,

that

its

correctness

is

not at all

self-evident.

It

includes the

proposition

that the chemical

effect of radiation

impinging on a body depends only on

the total

quantity

of the

acting radiation,

but

not

on

the

intensity

of the

radiation;

the existence

of

a

lower

action threshold for the

radiation

is

completely

ruled

out

by

this

assumption.

The

[4]

latter

puts us

in conflict with

the results of

two

papers by

E.

Warburg,3

which

stimulated

me

to

undertake the

present

work.

It follows from the

two

assumptions

that the number Z of the molecules of the

first kind that

decompose per

unit time

is

given by

the

expression

[3]

3E.

Warburg,

Verh.

d.

Deutsch.

Physik.

Ges.

9

(1908):

24 and

9

(1909):

21.