214

THEORY OF

SPECIFIC

HEAT

Doc. 38

PLANCK'S

THEORY OF

RADIATION

AND

THE THEORY

OF

SPECIFIC

HEAT

by

A.

Einstein

[Annalen

der

Physik 22( 1907): 180-190]

In

two

previous

papers1

I

have shown

that the interpretation

of

the

law

of

energy

distribution of

black-body

radiation

in

terms of

Boltzmann's

theory

of

the

second

law

leads

to

a

new

conception of

the

phenomena

of

light emission

and

light absorption,

which,

even though

still far

from

having

the character

of

a complete

theory,

is remarkable insofar

as

it

facilitates the understand-

ing

of

a

series

of

regularities.

The

present

paper

will

show

that the

theory

of

radiation-in

particular

Planck's

theory-leads

to

a

modification

of

the

molecular-kinetic

theory

of heat

by

which

some

difficulties

obstructing

the

implementation

of that

theory

can

be

eliminated.

The

paper

will also

yield

a

[2]

relationship

between

the thermal

and optical

behavior of solids.

First

we

will

give

a

derivation of the

mean

energy

of Planck's

resonator

that

clearly

demonstrates

its relation

to

molecular

mechanics.

To

that

end

we use a

few

results of the

general

molecular

theory

of

[3]

heat.1

Let

the

state

of

a

system

in the

sense

of the molecular

theory

be

completely

determined

by

the

(very

many)

variables

P1,P2...Pn.

Let

the

molecular

process

proceed according

to

the

equations

dPv/dt

=

Qv(P1,P2...Pn),

(v

=

1,2...n)

,

and

let the

relation

[4]

(1)D

0

V

hold for all values

of

the

Pv's.

[1]

1A.

Einstein,

Ann. d.

Phys. 17

(1905): 132 and 20

(1905):

199.