DOC.
47
287
the
same
kind.
With
respect
to
inertia,
a mass u
is equivalent
to
an
energy
content
of
magnitude
fic2.
Since
we
can
arbitrarily
assign
the zero-point
of
E0,
we are
not
even
able
to distinguish between
a
system's
"actual"
and
"apparent"
mass
without arbitrariness. It
seems
far
more
natural
to
consider
any
inertial
mass as a
reserve
of
energy.
According
to
our
result,
the
law
of the
constancy
of
mass
applies to
a
single physical
system
only when
its
energy
remains constant;
it is then
equivalent
to
the
energy
principle.
To
be
sure,
the
changes
experienced
by
the
mass
of physical
systems
during
the familiar physical
processes
are
always
immeasurably
small.
For
example,
the decrease in
mass
of
a system
that
gives
off
1000
gram-calories amounts to
4.6
x
10-11 gram.
The
radioactive
decay
of
a
substance is
accompanied
by
the release of
enormous
amounts
of
energy;
is the reduction
of
mass
in
such
a
process
not
large
enough
to be
detectable?
Mr.
Planck writes about this:
"According
to
J.
Precht1
1
gram-atom
of
[63]
radium,
if surrounded
by a
sufficiently thick
layer
of
lead,
releases
134.4
x
225
=
30,240
gram-calories per
hour.
According
to (17)
this
amounts
to
a
decrease
in
mass
of
30240-419-105
"
«
ha-«
" g.
iq'2
0
gr
-
1.41
x
10
mg
per
hour
or
0.012
mg
per year. Of course,
this
amount
is still
so
tiny,
especially in view
of the
high
atomic
weight
of
radium, that it
may
well
be
outside the
experimentally
accessible
range
for the
time being."
The
obvious
question
arises
whether
it
would
not be possible
to
reach
one's
goal
by
using
an
indirect
method.
If
M
is the atomic
weight
of
the
disintegrating
atom,
and
m1, m2,
etc.,
are
the
atomic
weights
of the
end
products
of radioactive
disintegration, then
we
must
have
M
-
S/w =
,
where
E
denotes the
energy
produced during
the disintegration of
one
gram–
atom;
this
can
be
calculated if the
energy developed
per
unit time
during
[65]
1J.
Precht,
Ann.
d.
Phys. 21
(1906):
599. [64]