208
BROWNIAN MOTION
mann
for
a
lack
of
emphasis on
the
comparison
of
his
theory
with
observation,[15]
Einstein
was firmly
convinced
of
the
principles
of Boltzmann's
theory.[16]
In his first
published
attempts
at
independent research,[17]
Einstein
took
for
granted
the
atomistic constitution
of
matter
and of
electricity.
He
developed
a
theory
of molecular
forces,
on
the
basis
of
which he established
a
number
of
relations
between observable
phenomena.[18]
Einstein's
interest
soon
shifted from
the
details
of
molecular forces to the
quest
for
facts,
"which
would
guarantee
as
much
as
possible
the
existence
of
atoms
of
definite finite
size"
("welche die Existenz
von
Atomen
bestimmter endlicher Grösse
möglichst
sicher stellten"),
as
he later characterized this
phase
of
his
work.[19]
III
The
irregular
movement
of
microscopic
particles suspended
in
a
liquid
had been
noted
long
before the botanist
Robert
Brown
published
his careful observations in
1828,[20]
but
he
was
the first to
emphasize
its
ubiquity,
and to exclude its
explanation as
a
vital
phenom-
enon.
Advances in observational
technique
and
in
theory
served to
eliminate
a
number
of
unsatisfactory
explanations
of
Brownian motion
by
the end
of
the
nineteenth
century,
if
not to
verify
the correct
one. Explanations
of
Brownian
motion
proposed
after the exclu-
sion
of
vital forces involved
capillarity,
convection
currents, evaporation,
interaction with
light, and electrical
forces.
During
the
1870s,
the kinetic
theory
of
heat
was
proposed
as
an
explanation by
several
authors.[21]
A
powerful argument
against
this
explanation
was
developed by
the
cytologist
Karl
von Nägeli
in
1879.[22]
He first used the
equipartition
theorem to calculate the
average velocity
of
the molecules
of
the
liquid,
and
then
used the
laws
of
elastic collision to obtain the
velocity
of
a suspended particle.
He
concluded that
the
velocity
of
such
a particle,
because
of
its
comparatively large
mass,
would be vanish-
ingly
small. William
Ramsay
and
Louis-Georges Gouy
independently
tried
to
defend the
molecular
explanation
of
Brownian motion
by assuming
the existence
of
collective
accounts
of
the
dispute, see
Brush
1976,
pp.
96-
98,
and Deltete
1983,
pp.
416ff.
[15]
On 30
April
1901,
Einstein
wrote to
Mi-
leva Maric:
"I
am now
studying
Boltzmann's
Gastheorie
once again. Everything
is
very
fine,
but too little value
is
placed on
the
comparison
with
reality"
("Ich studiere
gegenwärtig
wieder
Boltzmanns
Gastheorie.
Alles ist sehr
schön,
aber
zu wenig
Wert
gelegt
auf
den
Vergleich
mit
der Wirklichkeit")
(Vol.
1,
Doc.
102).
[16]
Einstein to Mileva
Maric, 13
September
1900
(Vol.
1,
Doc.
75).
[17]
For these
attempts, see
Einstein 1901
(Doc.
1)
and
Einstein
1902a
(Doc.
2);
for
a
dis-
cussion
of
the relevant
letters, see
Vol.
1,
the
editorial
notes,
"Einstein
on
Thermal. Electri-
cal,
and
Radiation
Phenomena,"
pp.
235-237,
and
"Einstein
on
Molecular
Forces,"
pp.
264-
266.
[18]
See Vol.
1,
the editorial note,
"Einstein
on
Molecular Forces,"
pp.
264-266;
and,
in this
volume, the editorial
note,
"Einstein
on
the Na-
ture
of
Molecular Forces,"
pp.
3-8.
[19]
See
Einstein
1979,
p.
44; translation,
p.
45. For
a
discussion
of
this shift
of
interest,
see
the editorial
notes,
"Einstein
on
the Foundations
of
Statistical
Physics,"
p.
46, and
"Einstein's
Dissertation
on
the Determination
of
Molecular
Dimensions,"
pp.
174-176.
[20]
See, e.g.,
Brown 1828. For
contemporary
reviews
of
Brownian
motion,
see
Smoluchowski
1906 and De
Haas-Lorentz
1913. For historical
accounts,
see Nye
1972
and
Brush
1968.
[21]
See
Brush
1968,
§
3.
[22]
See Nägeli 1879.