76

GENERAL MOLECULAR

THEORY OF

HEAT

of physical

system

for

which

we can

surmise

from

experience

that it

possesses

energy

fluctuation: this is

empty

space

filled with

temperature

radiation.

That

is,

if the linear dimensions

of

a space

filled with

temperature

radiation

are

very

large

in

comparison

with the

wavelength corresponding

to

the

maximum

energy

of

the radiation

at

the

temperature

in question, then the

mean

energy

fluctuation

will

obviously be

very

small in

comparison

with the

mean

radiation

energy

of

that

space.

In

contrast,

if the radiation

space

is

of

the

same

order

of

magnitude

as

that

wavelength,

then the

energy

fluctuation

will

be of

the

same

order

of

magnitude

as

the

energy

of

the radiation

of

the

radiation

space.

Of course,

one can

object

that

we

are

not

permitted

to assert

that

a

radiation

space

should

be

viewed

as a system

of the kind

we

have assumed,

not

even

if

the applicability of the

general

molecular

theory

is

conceded.

Perhaps

one

would have to

assume,

for

example,

that the

boundaries of the

space vary

with its

electromagnetic

states.

However,

these circumstances

need

not be considered,

as we

are

dealing

with orders

of

magnitude

only.

If, then,

in the

equation

obtained in the last section,

we

set

[26] ^

=

Jl

and

according to

the

Stefan-Boltzmann

law

[27]

E

=

cvT4

where

v

denotes the

volume

in

cm3

and

c

the

constant

of

this

law,

then

we

must

obtain for

3v a

value

of the order of

magnitude

of

the

wavelength

of the

maximal

radiation

energy

that

corresponds to

the

temperature

in

question.

One

obtains

=

Lji

=

0^2

?

where

we

have used

for

K

the value obtained

from

the kinetic

theory

of

[28]

gases,

and

7.06

x

10-15

for

c.