444

DOCS.

437,

438

JANUARY

1918

437. To

Pieter Zeeman

Berlin, 16 January 1918

Esteemed

Colleague,

Cordial

thanks

for

your

articles and for

the

commentary

thereon

in

your

letter,[1] as

well

as

for

your

wonderful earlier

papers on

the

ether-drag

coefficient.[2]

Among your

recent

investigations,

those

about the inertial

and

gravitational

mass

of

uranyl

nitrate

interest

me

most.[3]

Regarding

the

investigations

on

the

grav-

itation

and

inertia

of

crystalline substances,

the theoretical

background

to

the

problem

is

unfamiliar

to

me;[4]

even so,

I

understand that the

issues

can

be

addressed

without

such

theoretical

points of

view,

particularly

at

this time

of

revision of

the

general

foundations of

gravitation theory.

Cordial

regards, yours

very truly,

A.

Einstein

(This

letter

was

dictated

from

bed,

because

I

am

ill.)[5]

438.

To

Erwin Freundlich

[Berlin,

before

17

January

1918][1]

Dear

Freundlich,

Perhaps

it

is

better

if I

use

my personal

influence

concerning

that

instrument,

since

no

bellicose odium

weighs on

me.[2]

I

am on

the

Council of

the

Anti

Orlog

Raad,

you

know.[3] I

am

very pleased

that

you

are

being

received

so

amicably

at

Potsdam.[4] The ice

seems

finally

to

have broken.

The

notice

by

de Donder

is

scandalously superficial.

I

am

amazed

that

Lorentz

was

taken

in

by

it,

or

accepted

it

without

closer consideration.[5]

The

error

in

the

chain

of

reasoning

is,

as you yourself

indicated in

your

letter,

that

(16)

does

not

follow

from

(15).

(15)

is

not

an

identity

but

an

equation of

condition.

The

deduction

is

approximately

thus.

Assertion:

No circle

can

exist

on a

plane.

Proof:

The circle

equation

x2

+

y2

=

R2

is

intrinsically inconsistent;

for since

the

right-hand

side

is independent

of

x,

the

left-hand

side

must

also be.

The

equation

is

therefore inadmissible!