4
NATURE
OF
MOLECULAR FORCES
force
with
a
range
so
short that the
force
vanishes
at
microscopic
distances. On the basis
of
this
assumption,
Laplace was
able
to
explain
the existence
of
surface tension and several
other
capillary phenomena.[7]
His
theory
of
capillarity was
elaborated
by
Poisson
and
Gauss,
among
others.
Such
static molecular models
could
not
be
used
for
the
investigation
of
the thermal
aspects
of
capillary phenomena.
In
the latter
part
of
the
nineteenth
century
a strictly thermodynamical approach
to
capillarity was
developed,
which relates
various
capillary
phenomena
without
any assumptions
about the
underlying cause
of
the
assumed
surface tension. The concurrent
successes
of
the kinetic
theory
of
gases
led to
attempts
to
develop
a
kinetic
theory
of
liquids,
and thus to
a
revival
of
interest in the
molecular
theory
of
capillarity.
Van
der
Waals,
in
particular,
used
the
study
of
capillarity as
a
way
of
inves-
tigating
molecular cohesive
forces.[8]
His
work
was
the first to combine the methods
of
the
Laplacian theory
with
those
of
kinetic
theory.
Maxwell, Rayleigh,
William Thomson, and
Boltzmann
are
other
leading physicists
who
worked
on
the
theory
of
capillarity
during
the
last
quarter
of
the nineteenth
century.
However,
no
substantial
progress
toward
a
kinetic
theory
of
liquids was
made.
A fellow ETH student later
reported
that Einstein
was very impressed by a
lecture
on
capillarity
given by
Minkowski
during
Einstein's
last term at the
ETH
(April-July
1900).[9]
In
September
1900 he finished
reading
Boltzmann's
Gastheorie, which
contains
a
discussion of
capillarity,
based
on
Van der
Waals's
approach
that
may
have
further
stim-
ulated
Einstein's
interest.[10]
The first evidence that he
was actually
at work
on a theory
of
molecular forces dates from the
following
month. In
a
letter to Mileva Maric he
refers
to
results
on
capillarity
that
he
had
recently
found.[11] He believes the results to be
completely
new,
and
suggests
that
they
look
for
empirical
data
to test them.
"If
a
law
of
nature
re-
sults"
("wenn sich
dabei ein
Naturgesetz
ergibt"),
he
wrote, a
paper
would
be submitted
to the Annalen
der
Physik.
Two months
later, on
16 December
1900,
the
manuscript
of
Einstein
1901
(Doc.
1)
was
received
by
the Annalen.
Einstein studied the
dependence
of
capillary phenomena,
and hence
of molecular
forces,
on
the chemical
composition
of
neutral
liquids.
Like
many
contemporary
treat-
ments
of
capillarity,
Einstein's
paper
combines the
use
of
both
thermodynamic
and
mo-
lecular-theoretical
methods. The
paper
starts with
an
ingenious
thermodynamical
argu-
ment,
based
on
the
experimentally observed, approximately
linear
decrease
in surface
tension with
increasing temperature,
to
prove
that the surface
energy
of
a
fluid
consists
almost
exclusively
of
potential energy.
Einstein then calculated this surface
potential
en-
ergy on
the basis
of
an assumption
about the nature
of
molecular forces. Guided
by a
[7]
See
Laplace
1806.
[8]
See
Van
der
Waals 1873. For discussions
of
Van der
Waals's
work,
see
Rowlinson
1973;
Klein 1974a.
[9]
See Kollros
1956,
p.
21. Minkowski
was
already
at
work
on
his review article
on capillar-
ity,
Minkowski 1907a.
[10]
See Einstein
to
Mileva
Maric, 13
Septem-
ber and
19
September
1900
(Vol. 1,
Docs. 75
and
76).
Boltzmann discusses Van der
Waals's
theory
of
fluids and its
relationship
with La-
place's
theory
of
capillarity
in
sec.
I
of
Boltz-
mann
1898a. Einstein
sent
a manuscript copy
of
his first
paper
to Boltzmann
(see
Mileva Maric
to Helene
Savic,
20
December
1900,
Vol.
1,
Doc.
85).
[11]
Einstein to Mileva Maric, 3
October 1900
(Vol. 1,
Doc.
79).
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