8
NATURE OF
MOLECULAR FORCES
deprecatingly
about his two
papers
on
molecular
forces. He sent Stark
a
collection of
his
offprints, omitting
"my two worthless
beginners'
works"
("meine zwei
wertlosen
Erstlingsarbeiten").[36]
Einstein
published one
final
paper on capillarity
and molecular
forces,
Einstein
1911a.
In this
paper,
he
demonstrated,
without
mentioning
his earlier
papers,
that
the
assumption
of
a
universal
range
for the molecular cohesive force
is
incompatible
with certain
empiri-
cally
established laws
obeyed by
the surface tension. He stressed the need
to
introduce
a range
for the force between two molecules that
is
dependent on
the nature
of
the
molecules.
Einstein
1902a
(Doc.
2) opens
with
a
discussion
of
the conditions for the
validity
of
the
second law
of
thermodynamics, a
discussion that
proved
to be
of
some significance
for
Einstein's
work. He
points
out that the theories
of
dissociation and
of
dilute
solutions
are
based
on
the
application
of
the second law
of
thermodynamics
to
processes
involving
idealized
semipermeable
membranes that
can separate any
two
(or more)
substances.
Al-
though even
the
approximate realizability
of
such
processes
is
often
doubtful, the
predic-
tions
of
the theories
are
confirmed
by experiment.[37]
It
seems,
therefore,
that
one can
draw
valid conclusions from the consideration
of
highly
idealized
processes.
In
particular,
it
seems
that
one may
apply
the second
law
to
mixtures acted
on by
external
conservative
forces,
which
produce
the
same
effects
as
semipermeable
membranes.
Generalizing
this
conclusion, Einstein formulated the
hypothesis
that
the
second law
may
be
applied
to
mixtures, the
components
of
which
are subjected
to
arbitrary
conservative
forces. The
arguments
in the
body
of
the
paper
make extensive
use
of
this
hypothesis.
Einstein's
next
paper,
which initiated his
study
of
the
statistical foundations
of
thermodynamics,
may
have
been
stimulated
by
the
need he
felt
to
justify
this
hypothesis.
In
any
case,
the
final
section
of
that
paper
is devoted
to
providing
such
a
justification.[38]
[36]
Einstein
to Stark, 7
December 1907. He
did not mention his
theory
of
molecular forces
in his Autobiographical Notes
(see
Einstein
1979).
[37]
See
Planck
1891 for
a
discussion of
the
role
of
ideal
processes
in
thermodynamics.
Nernst
1898,
pp.
102-103, discusses
the
ther-
modynamical
use
of
idealized
semipermeable
membranes.
See also
Einstein
1902a
(Doc. 2),
note 1.
[38]
See Einstein 1902b
(Doc. 3),
p.
433, and
the editorial
note,
"Einstein
on
the Foundations
of
Statistical
Physics,"
p.
46.
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