MOLECULAR FORCES
265
1806).
Einstein wrote
Ostwald that
his work
on
this
topic
had been
stimulated
by
Ostwald 1891
(see
Doc.
92).[3]
In
Einstein's
theory,
the
force
between
two
molecules
is
derived from
a
potential:
P
=
P"-c
c'0(r),
where
c
and
c'
are
constants
characteristic of
the two molecules,
and
/(r)
is
a
universal
function of the distance between them. Einstein
was
intrigued
by
the
possibility
of
an
"innere Verwandtschaft" between this
force
and
gravitation
(see
Docs.
100,
101).[4]
He continued
to
work
on
capillarity
after
publication
of
his first
paper,
but
apparently
without
success (see
Doc.
119).
By
the
spring
of
1901,
Einstein had
applied
his
theory
of molecular
forces to
salt
solutions. He studied
infinitely
dilute solutions in order
to
avoid the
complications
introduced
by
interactions between solute molecules
(see
Doc.
101).
The results of
this
investigation
were
published
in
Einstein 1902a.
By
applying
the
theory
to
mixtures of neutral
liquids,
he
drew the conclusion
that,
for
the molecules of such
a
liquid,
c
should
be
proportional
to
the
specific
volume. If
this result
were
verified
experimentally,
he
wrote,
it
would
mean
the end of the
mo-
lecular-kinetic
theory
of
liquids
(see
Doc.
127).[5]
Einstein also
attempted
to
extend
his
theory
of molecular
forces
beyond
liquids;
he
planned
to
use
the results
as
the basis of
a
doctoral dissertation
(see
Doc.
100).
In
September
1900,
after
studying
Boltzmann's work
on
gas theory,
he
stated that
"die
hypothetischen
Kräfte" between molecules
play
no
essential role
in
the kinetic
theory
of
gases (see
Docs.
75,
76).
The
following year
he
realized that these
forces
play
a
role
in
transport phenomena
in
gases.[6]
He
set out to
evaluate the
coefficients
of
diffusion,
heat
conduction,
and internal friction
in terms
of
his constants
c (see
Doc.
101).
In this
connection,
Einstein
was
troubled
by
the need
to
take into
account
a
finite
molecular
size in
addition
to
the central
force law
between molecules
(see
Doc.
102).[7]
[3] There
is
reason
to believe
that Minkow-
ski
was a more
important
influence.
He
gave
a
lecture
on
capillarity during
Einstein's last
term at
the ETH. Einstein
is
said
to
have
re-
marked,
"Das
ist die erste Vorlesung
über
mathematische
Physik,
die
wir
am
Poly
hören!"
(Cited
by
Louis Kollros
in
Seelig
1956,
p. 21.)
[4] Ostwald attributes the idea
of such
a re-
lationship to
van't Hoff
(see
Ostwald
1891,
p.
1142).
A
similar
relationship is suggested
in
a
popular-scientific
book
(Bernstein
1870,
pp.
142-143)
that Einstein
read
in his
youth
(see
Einstein
1979,
pp. 12, 14).
[5] This
is
probably
a
reference to
the
van
der Waals
theory
(see
Doc.
127,
note
16).
[6]
See Boltzmann
1896,
§§12,
13.
Following
Maxwell's
analysis,
Boltzmann showed that
the
difficult
mathematical
problems
connected
with
the
study
of these
phenomena
could
be
solved
for
one
particular
force law (inverse
fifth
power).
[7] The
van
der Waals
theory
of
gases
and
liquids
is
based
on
the
assumption
that mole-
cules
have
a
finite extension
as
well
as
an
attractive central
force
between them
(see
the
first two
sections of
Boltzmann
1898).
The
failure
to
introduce
a
characteristic
length for
each
molecule
(molecular "radius")
is
the
fun-
damental
difficulty
in Einstein's
theory
of
molecular
forces
(see
the editorial
note
pre-
ceding
Einstein
1901 in
Volume
2).
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