DOC.
5
75
E
"00
0
2k
P
EE
-
P
2
KT
e
uiEdE
=
0
.
[23]
This
equation states
that the
mean
value
of
the bracketed
expression
vanishes,
and hence
2kP
U
=
P
-
EE
.
[24]
In
general,
the instantaneous value
E
of
the
energy
differs
from
E
by a
certain
amount,
which
we
call
"energy
fluctuation";
we
put
E
=
E
+
6
.
We
then obtain
P
-
EE
=
e*
=
2KP
Ü
.
[25]
The
quantity
e2
is
a
measure
of the thermal stability of the
system;
the
larger
the
e2,
the less this stability.
Thus
the absolute
constant
K
determines
the thermal
stability
of the
system. The
relationship
just
found is interesting because it
no
longer
contains
any
quantity
reminiscent
of
the
assumptions
on
which the
theory
is
based.
The magnitudes
of
e3,
e4, etc.
can
be
calculated
by
successive differ-
entiations
without
any
difficulty.
§5.
Application
to
radiation
The
last-found
equation
would
allow
an
exact
determination of the uni-
versal
constant
k
if it
were
possible
to
determine the
mean
value of the
square
of the
energy
fluctuation of
a
system;
however,
at
the
present state
of
our knowledge
this is
not
the
case.
In
fact,
there is
only
a
single kind