272 DOC.

23

MAX PLANCK AS SCIENTIST

specific thermodynamical questions.

However,

we

should

not

leave

unmentioned

Planck's

polemical paper,

"Gegen

die

neuere Energetik" ["Against

the New

Energetics"],

which

was

published

in 1896

in

Wied.

Ann.,

vol.

56,

because it

undoubtedly

had

a

significant

influence

on

those

working

in this

area.

This is

a

masterfully

written short article that shows that

energetics

is useless

as a

heuristic

method, indeed,

that it

operates

with untenable

concepts.

To

every

friend

of

lucid

scientific

thinking,

the

reading

of this fresh little article

can

make

up

for the

annoyance

that he

probably

could

not

suppress

when

reading papers

of

the kind here

combatted.

In the

year

1896

Planck turned

to

the

theory

of radiation. It is

common

knowledge

that his

studies in this

area

had

a

powerful

influence

on

the current

development

of

physics.

The

great

advances that have been made in the

theory

of

heat

during

the

past

few

years

would have

scarcely

been

accomplished

without these

studies. That whole

complex

of

results,

theoretical

conceptions,

and formulations of

problems

that

comes

today

to

the

physicist's

mind when he hears the word

"quanta,"

that enlivens his existence and makes it

so

difficult

at

the

same

time,

has arisen from

these studies. In

order

to

appreciate

Planck's

accomplishments

in this

area,

we

must

cast

a

fleeting glance

on

the

development

of the

theory

of

radiation.

Every body gives

off heat radiation. As

a

consequence,

a

cavity

contained in

a

nontransparent body

is

constantly permeated by

thermal radiation. On the basis of

simple thermodynamic arguments,

Kirchhoff found in the sixties

of the last

century

that this radiation

must

be uniform in all directions and that its

properties

can

depend

on

nothing

whatsoever other

than the

temperature

of the

body enclosing

the

cavity.

Thus,

if

we

denote

by

u

dv

the

energy

of

radiation from

the

frequency region

dv contained in

a

unit

volume,

then

u (the

monochromatic radiation

density) depends only

on

the absolute

temperature

T

and the

frequency

v,

while it is

totally independent

of

the

physical

and chemical

nature

of the

walls that enclose the

cavity.

Thus,

u

(v,

T)

is,

to

use

the

customary

expression,

a

universal

function of the

two

variables

v

and

T,

and

its

determination

is

the

most

important

experimental

and

theoretical task

of the

theory

of radiation. At

first,

nothing

could be found

out

about this function in

a

purely thermodynamical

way.

The

next

theoretical advance

was

made in

1884

by

Boltzmann,

who showed that

one can

deduce the law for total radiation

density,

oof0

udv

=

oT4

,

by thermodynamic means

if

one

bases oneself

on

the law of radiation

pressure

that

Maxwell deduced from the

electromagnetic theory

of

radiation, according

to which

[10]

[11]

[12]

[13]

[14]

[15]