EINSTEIN ON THE
NATURE
OF
MOLECULAR FORCES
Einstein's first two
papers
have
a common
theme:
an investigation
of
the nature
of
molec-
ular
forces
by
means
of
the effect
of
such forces
on
various observable
phenomena
in
liquids.
Einstein
1901
(Doc.
1)
concentrates
on
capillary phenomena
in neutral
liquids;
Einstein 1902a
(Doc.
2)
deals with dilute salt solutions.
Popular-scientific
books
and school texts
no
doubt
familiarized Einstein
at
an early age
with the
general
ideas
of
the atomic-molecular
theory
and the
concept
of cohesive
forces
between molecules.[1]
Violle's
textbook
of
physics,
which Einstein studied in
1895,
argues
strongly
for the molecular
outlook,
and contains
a lengthy chapter on
capillarity,
based
on
Laplace's
theory
of
short-range
forces between the molecules
of
a
fluid
(see below).[2]
Mach's
Mechanik,
which Einstein read about
1897,
also introduces such forces to
explain
the
shape
of
fluid
bodies.[3]
A record
of
his nascent interest
in
studying
the nature
of
these
forces dates
from Einstein's ETH
years.
His notes
on
Weber's
physics
lectures contain
the
marginal
comment "Investigate! Holidays." ("Untersuchen! Ferien.") next to his
sum-
mary
of Weber's
comment
on
the unknown
function
of
the distance
between
two identical
molecules that characterizes
the cohesive force between
them.[4]
In his first
published paper,
Einstein
1901
(Doc. 1),
Einstein chose the
phenomenon
of
capillarity
to
study
the nature
of
intermolecular forces. Before
him,
many
physicists
had
sought
to
explain capillarity on
the basis
of
cohesive forces
acting
between the
constituent
particles
of
matter.[5]
Laplace
had
proposed an
ambitious
program
for
unifying physics by
postulating
the existence
of
various central forces between molecules in
order
to
explain a
number
of
physical phenomena.[6]
The most detailed
and
successful
example
of
this
ap-
proach
is
his
theory
of
capillarity,
based
on
the
hypothesis
of
an
attractive intermolecular
[1]
These
concepts are
mentioned in both
Bernstein
1853-1857
and
Büchner 1855
(for
evidence that Einstein read these books
as a
boy,
see
Vol.
1,
"Albert
Einstein-Beitrag für sein
Lebensbild,"
p. lxii).
The
physics
text
that Ein-
stein used
during
his last
term at
the
Luitpold
Gymnasium
(Krist 1891;
see Luitpold-Gymna-
sium,
Curriculum, Vol.
1, Appendix
B,
p.
353),
discusses the molecular structure
of
matter
(ibid.,
p.
5)
and forces between
molecules,
in-
cluding
cohesive forces (ibid.,
pp.
13-14).
[2]
Einstein used
Violle's
textbook
to
prepare
for the ETH entrance examination
(see
Vol.
1,
"Albert
Einstein-Beitrag
für sein Lebens-
bild,"
p.
lxiv).
Violle 1892
argues
that
"the
as-
sumption
of
molecules is
...
not
just a hypoth-
esis, but the
simple expression
of
a
fact"
("Die
Annahme
von
Molecülen ist
...
nicht
nur
eine
Hypothese,
sondern der einfache Ausdruck ei-
ner
Thatsache")
(p. 339).
The section
on capil-
larity,
Violle 1893,
chap.
III, pp.
575-663,
in-
cludes
extensive
historical,
experimental,
and
theoretical discussions.
[3]
See
Mach
1897.
Mach's
discussion
of
mo-
lecular
forces is in
sec.
10
of
chap.
III. For evi-
dence that Einstein read
Mach's
Mechanik,
see
Einstein
to
Mileva Maric,
10
September
1899
(Vol.
1,
Doc.
54),
especially
note
8.
[4]
Vol.
1,
Doc.
37,
p.
130. Einstein took the
course
notes
during
the winter
semester
of
1897-1898, but it is
possible
that
some
of
his
marginal comments
were
added later. See Vol.
1,
the editorial note,
"Einstein
on
Molecular
Forces,"
pp.
264-266, for
a
discussion
of
Ein-
stein's
early
interest in this
subject.
[5]
For reviews
of
eighteenth
and
nineteenth–
century
theories
of
capillarity, see
Minkowski
1907a; Weber,
R. H.
1916; Bikerman
1975,
1978;
Rowlinson
and
Widom
1982,
chap. 1;
Rüger
1985.
[6]
For
Laplace's program,
see
Fox 1974.