DOC.
38
219
tions between
the
current
molecular-kinetic
theory
and
experience
must
be
expected in other
areas
of the
theory
of heat
as
well,
which
can
be
resolved
along
the lines indicated.
In
my
opinion
this is
actually the
case,
as
I
shall
now
attempt to
show.
The simplest conception
one can
form
about thermal
motion
in solids is
that its individual
atoms perform
sinusoidal oscillations about
equilibrium
positions.
With
this
assumption,
by
applying
the molecular-kinetic
theory
[17]
(equation
(4))
while
taking
into
account
that three
degrees
of
freedom of
motion must be
assigned
to each
atom,
one
obtains for the
specific heat of
a
gram-equivalent
of
the substance
c
=
3Rn
,
or-expressed in
gram-calories–
c
=
5.94
n
,
when
n
denotes the
number
of atoms in
the molecule. It
is well
known
that
this relation
applies
with
remarkably
close
approximation
to most
elements
and
to
many
compounds
in
the solid
aggregation state
(Dulong-Petit's
law,
rule
of
F.
Neumann
and
Kopp).
However,
if
one
examines
these facts
a
little closer,
one
encounters two
difficulties that
seem
to
set
narrow
limits
on
the applicability
of the
molecular
theory.
1. There
are
elements
(carbon, boron,
and
silicon)
that in
the solid
state
and at ordinary temperatures
have specific atomic heats
much
smaller
than 5.94. Furthermore, the
specific
heat
per gram-molecule
is less than
[19]
n.
5.94
in all solid
compounds
containing
oxygen,
hydrogen
or
at
least
one
of
the elements
just
mentioned.
[20]
2.
Mr. Drude
has
shown1
that the
optical
phenomena
(dispersion)
lead
to
the conclusion that several
elementary
masses
moving
independently
of each
other
must
be
ascribed
to each
atom
of
a compound
in
that
he
successfully
1P.
Drude, Ann. d.
Phys. 14
(1904): 677.
[18]
[21]