BROWNIAN
MOTION
209
motions
of
large
numbers
of
atoms in
liquids, an assumption
suited to the
refutation of
arguments
such
as Nägeli's.[23]
In 1900
an entirely
different
way
of
applying
the kinetic
theory
of
heat
to
Brownian
motion
was investigated by
Felix
Exner,
who assumed
an equipartition
of
energy
between
the molecules
of
the
liquid
and the
suspended
particles.[24]
He calculated the
velocity
of
the molecules
on
the basis
of
observations that he
interpreted
as giving
the
mean
velocities
of
the
suspended particles, obtaining
results that
were
not
in
agreement
with
contemporary
estimates
of
molecular velocities. In
Exner's
work
there is
no
fundamental
difference
be-
tween
a
solute molecule and
a suspended particle.
Einstein arrived at
a
similar
conclusion,
but instead
of
emphasizing
the
equipartition
theorem,
he took the osmotic
pressure
and
its
relation to the
theory
of
diffusion and to the molecular
theory
of
heat
as
the
starting point
of
his
analysis
of
Brownian motion:
According
to this
theory, a
solute molecule differs from
a suspended body solely
with
regard
to
magnitude,
and it
is
not
apparent why
the osmotic
pressure
of
a
number
of
suspended particles
is
not the
same as
that
of
the
same
number
of
solute molecules.
Nach
dieser
Theorie
unterscheidet sich
eingelöstes
Molekül
von
einem
suspen-
dierten
Körper
lediglich
durch die
Größe,
und
man
sieht nicht ein,
warum
einer
Anzahl
suspendierter
Körper
nicht derselbe
osmotische Druck
entsprechen
sollte,
wie der nämlichen Anzahl
gelöster
Moleküle.[25]
On the other
hand,
Einstein
pointed
out,
according
to the
"classical
theory
of
thermo-
dynamics"
("klassischen Theorie der
Thermodynamik"),[26] suspended
particles-as
macroscopic
objects-should
not
exert
an
osmotic
pressure on a semipermeable
wall.
Be-
fore Einstein,
no one seems
to have
recognized
that this contrast
provides a
touchstone
for
the kinetic
theory.
His choice
of
a suspension
to
study
the relations between the thermo-
dynamic
and atomic theories
of
heat amounted to
a
radical reversal
of
perspective. Usually
the
legitimacy
of
microscopic explanations
of
thermodynamic
results
was
at issue. In
this
case, however,
the
question
centered
on
the
applicability
of
a thermodynamic
concept–
osmotic
pressure-to the
suspended particles.
In the
course
of
studying
colloidal
solutions,
the
commonly
made
distinction between
suspensions
and solutions in
nineteenth-century
chemistry
had
gradually
lost its
absolute
character.[27]
The absence
of
any
fundamental difference between solutions and
suspen-
sions
was
made
strikingly
clear in
1902,
when observations
performed
with the
newly
[23]
See
Ramsay
1882 and
Gouy
1888.
[24]
See
Exner
1900.
[25]
Einstein
1905k
(Doc. 16),
p.
550. The
at-
tempt
to
use
osmotic
pressure as a distinguishing
feature
of
solutions
was
also
rejected by Zsig-
mondy
(see
the introduction
to
Zsigmondy
1905).
[26]
Einstein
1905k
(Doc. 16),
p.
550. For
a
discussion
of
what Einstein meant
by
classical
thermodynamics,
see Nye
1972,
p.
139,
fn. 52.
[27]
For
a contemporary
discussion
of
the dis-
tinction between solution and
suspensions, see
the introduction
to
Zsigmondy
1905. For
a
dis-
cussion
of
colloidal
chemistry
and its relation
to
the
study
of
Brownian
motion,
see Nye
1972,
pp.
98-102.