206

MOTION

OF CENTER

OF GRAVITY

_

Z

(y^

+

I

%

+

PedT

xpedr

where, according to

the

energy

principle, the value of the denominator

on

the

right-hand

side

is

independent

of

time.1

Hence

we

may

write

equations

(2b)

also in the form

(2c)

de/dt

=

const.

Thus,

if

one

ascribes the inertial

mass

E/V2

to

any energy

E,

then-at least in first

approximation-the principle

of conservation

of

the

motion

of

the

center

of

gravity also

holds for

systems

in

which

electro-

magnetic processes

take place.

The

present investigation

shows

that

one

either

has

to give

up

the

fundamental

law

of mechanics,

according

to which

a body

originally

at rest

cannot perform

translational

motion

unless acted

upon by

external

forces,

or

one

has to

assume

that

a

body's

inertia

depends

on

its

energy

content

according

to

the

law

stated.

Bern,

May

1906.

(Received

on

17

May

1906)

1According

to the

interpretation

developed

in this

paper,

the principle

of

the

constancy

of

mass

is

a

special

case

of the

energy

principle.