226 DOCS.
230,
231
JUNEJULY
1916
230. To
Théophile
de
Donder
Berlin,
13
Wittelsbacher
St.,
30
June
1916
Esteemed
Colleague,
Many
thanks
for
sending
me
your
interesting analysis.[1]
Our
lack
of
agreement
regarding
the
energy
components
for
the
gravitational field
surely
arises from
your
determination
C =
0,
which
is
always
inconsistent with
the
gravitation
field
equations
when
the scalar
of
the
mass
energy
tensor does not
vanish.[2]
This
can
be
seen
by
forming
the
appropriate
scalar
equation
out
of the
field
equations
through
inner
multiplication
by
guv.
It
is
not
correct
that
in
my
formula
(2a),
p.
845 of
my
pap[er]
Sitz. Ber. XLVIII
1915,[3]
a
Vg factor
is
missing.
On
both
sides
of
the
equation
a
covariant
tensor
of second order must
appear;
a
Vg factor
on
the
lefthand side would
destroy
the
equations’
invariant character.[4]
I
have
presented
the
entire
theory
in
summary
in
a
paper
appearing
in
the
most
recent
Annalen.[5]
In this
paper you
can
also find
a
simplified
description
of
vacuum
electromagnetics,
in which it
is
shown
that there
is
no
need for
the
“dual” sixtensor
[Sechservektor] concept.[6]
Very respectfully yours,
A Einstein.
231.
From Théophile de Donder
[Brussels,] 4
July 1916
S[ir]
and
esteemed
Coll[eague]
(Einstein)
I
thank
you

postcard
[of]
30
June
1916.[1]

exact
meaning
of
your symbols.
I.
May
I
ask
you
to
consider first
the
case
of
the
elect[rical]
vacuum
permeated
by
electr[icity]
but
devoid
of
actual
matter,
it
is
exclusiv[ely]
to such
a
field
that
my
note relates. In
appl[ying]
Hamilton’s
gener, principle
I
find
the
11 eqs.;
(330,
352, 353,
ch.
VII);[2]
by means
of
the
identity[3]
{I
+
£im)irnl
=
k(~9)^Y2Yl9kl(ik, lm)

k(g)^Cgim,
k
i