DOCS.
275,
276
AUGUST
1911
197
const. But his
expression
is not
correct if
permanent magnetization
is
involved.
There
is
no
solution that
would
encompass
all
cases.
(iH)
was
correct
in
our
paper.
In
Abraham's
paper
(iB)
is correct.[8]
What
was
perhaps open to
criticism in
our
paper
was
our
introduction of
angular momenta.
One
can
do that,
but
it
is not required,
and
the
treatment
becomes
simpler
when
angular
momenta
are
excluded.
The
relativistic
treatment
of
gravitation is
causing
serious
difficulties.
I
consider
it
probable
that
in its
customary
formulation
the
principle
of
the
constancy
of the
velocity
of
light
holds
only
for
spaces
of
constant
gravitational
potential.[9]
Best
regards
to
you
and
your wife[10]
from
your
A.
Einstein
My
wife also
sends her best
regards.
Lenard[11]
and
his
pals are
and
remain detestable
swine.
276. To
Michele
Besso
[Prague,
second
half of
August
1911][1]
Dear
Michele,
Here
are
the
collars and
the booklet for
Vero.[2]
From the latter
you
can see
what
an
inveterate schoolmaster
I
have
already
become. I
am
still
scribbling
about
the
Boltzmann
principle.
It
seems
that the
principle
really
does hold
exactly
if
the
entropy
is
conceived
as
the
property
of
a
state
region.[3]
For
each
state
the
entropy
becomes
-*.
For
example,
to
bring
a
suspended particle
into
a
certain
layer a, one
has
to
perform
great
work
against
the
osmotic
pressure
of
this
particle.[4]
One
can see
in this
example
that
the
Boltzmann
equation
holds
exactly.
Unfortunately,
we
lack
the
knowledge
required
to
determine the
entropy
in
a
general
way
while
taking
into consideration the
statistical
irregularities,
so
that
we
must content ourselves with
an
approximation.
It
is
also
embarrassing
that
we can only
speak
of the
entropy
of
such states
that
are
at
least
in
principle thermodynamically
realizable.
It
is
thus
always
necessary
to contrive
an
analogue
of the
semipermeable
membrane,
which, however,
in most
cases
cannot be
accomplished.
We
cannot
conceive
of
"walls"
that
will
keep
the
thermal
energy
of
a
subsystem
within
specific
limits.
For that
reason,
the
entropy
of
a
heat
content
region
cannot be
determined
either,
so
that
the calculation
of
the
temperature
fluctuation lacks
rigor.
The situation
is just
as
bad
when it
comes
to
the
energy
of
a
periodically
oscillating system.
Some
means
ought
to be
found
for
extending
the
entropy concept
of
thermodynamics
to
cases
that
cannot be
regarded
as
instances
of
thermodynamic
equilibrium.
I
hope
that
you
will
use
what
is
left
of Vero's vacation
to
make
some
proper
excursions. I
am glad
that
all
of
you are
cheerful
and
in
good
health. After several
attempts
to find
a
better shelter
for
Fanni's
child,
we
took
it
into
our own
house
today.[5]