DOC. 20 THEORETICAL ATOMISM 235
Obviously, a theory
acquires
scientific value
only
insofar
as
the
assumptions on
which
it is
based
are
simpler,
i.e.,
less
numerous,
than those of their
consequences
that
can
be
compared
with
experience.
In
addition
to
the molecular
hypothesis,
kinetics also
uses
the
assumption
that
it
is
permissible
to
apply
the laws of mechanics
to
molecules and
to atoms
without
making
any
changes,
the
atoms
being
treated
as
material
points.
The latter
means
that
the
position
of
an
atom
can
be conceived of
as
being
determined
through
the
specification
of
a
single point,
i.e.,
that
one
cannot
speak
of
an
orientation
or
rotation
of
an
atom.
We imagine now
an
arbitrary isolated physical system of bodies, i.e., one that
does not interact
in any
way with bodies of other systems. In accordance with the
theory, this system consists of
an
extremely large number of atoms which exert
certain forces on each other that depend only
on
their position, and which move
according to the laws of mechanics. If we followed an atom on
its
path for
a
while,
we
would observe
that,
due
to
interactions with other
atoms,
its
velocity
c
takes
on
c2
the
most
diverse values
over
the
course
of
time,
and
thus
also the
quantity
m
-
(m
=
mass
of the
atom),
which
is
designated as
the atom's kinetic
energy.
But if
we
follow the
atom
for
a
sufficiently
long
time,
we
will be able
to
form
a mean
value,
which
we
denote
by
"L,"
from
all
of the values that the kinetic
energy
assumes over
the
course
of
time. The
analysis yields
now
quite generally
the
proposition
that this
c2
time
average
L of
the
variable
quantity
m
-
has
the
same
value
for
all
of
the
atoms
of
the
system.
One has
to
conceive of
a
molecule
as
consisting
of several
atoms,
which,
even
though
movable relative
to
each
other,
are
endowed with forces
owing
to
which the distances between the
atoms
that form the
molecule cannot increase
beyond
certain limits. At
every moment,
the
center
of
gravity
of
a
molecule has
a
certain
velocity
C,
which
is
determined
by
the velocities of
its
atoms;
C
is
called,
fittingly,
the
velocity
of the
translatory
motion of the molecule. If
M is
the
mass
of
C2
the
molecule,
then
M
-
can
be called the kinetic
energy
of the
translatory
motion
of the molecule.
Analysis
shows that the
time
average
of the latter
is
also
equal
to
L,
i.e.,
that it
is
identical for
all
molecules of the
system,
and that
it
equals
the
corresponding average
value for the individual
atom.
Thus,
the
quantity
L
is
a general
measure
of
the
intensity
of
the
molecular motion
in
a
system.
If
two
initially
isolated
systems
have the
same
L
and
are
then combined
into
a
single system
without work
or
heat
having
been
supplied
to
the
system
(contact),
the characteristic
quantity
L
of the total
system
is
the
same as
that of the
two
original
systems;
no energy exchange
takes
place upon
the
contact. However,
if
A
General Re-
sult
of
the
Theory
(Equipartition
of the Kinetic
Energy).