172
DISSERTATION
ON
MOLECULAR
DIMENSIONS
or
ions
of
the
solute.[19]
Einstein's
dissertation contributed to the solution
of
this
prob-
lem.[20]
He recalled in 1909:
At the time
I
used the
viscosity
of
the solution to determine the volume
of
sugar
dissolved in water because in this
way
I
hoped
to take into account the volume
of
any
attached
water molecules.
Ich habe seinerzeit
zur Bestimmung
des Volumens des in Wasser
aufgelösten
Zuckers
deswegen
die
Viskosität
der
Lösung benutzt,
weil
ich
so
das Volumen
eventuel angelagerter Wassermoleküle mit
zu
berücksichtigen hoffte.[21]
The results obtained in his dissertation indicate that such
an
attachment does
occur.[22]
Einstein's
concerns
extended
beyond
this
particular question
to
more general
problems
of
the foundations
of
the
theory
of
radiation and the existence
of
atoms. He
later
empha-
sized:
A
precise
determination
of
the size
of
molecules
seems
to
me
of
the
highest
importance
because
Planck's
radiation formula
can
be tested
more
precisely
through
such
a
determination than
through
measurements
on
radiation.
Eine
präzise Bestimmung
der Grösse der Moleküle scheint mir deshalb
von
höchster
Wichtigkeit,
weil durch eine solche die
Strahlungsformel von
Planck
schärfer
geprüft
werden kann als durch
Strahlungsmessungen.[23]
The dissertation
also
marked
the first
major success
in
Einstein's
effort
to find
further
evidence
for
the atomic
hypothesis,
an
effort that culminated
in
his
explanation
of
Brown-
ian
motion.[24]
By
the end
of
1905 Einstein had
published
three
independent
methods
for
determining
molecular
dimensions,
and
in
the
following years
he found several
more.[25]
[19] Bousfield 1905b,
a study
of
the relation-
ship
between
the sizes
of
ions and the electrical
conductivity
of
electrolytes,
calls this the most
important open problem
of
the theory
of
aqueous
solutions
(p. 257).
For
contemporary
reviews
of
research
on hydrates, including a history
of
this
problem, see
Washburn 1908 and
1909,
and
Dhar
1914.
[20]
Einstein
1906a is cited in Washburn
1909,
p.
70,
and in
Herzfeld 1921,
p.
1025,
as provid-
ing
evidence
for the existence
of
an
association
between
a
solute molecule and molecules
of
the
solvent.
[21]
Einstein to
Jean
Perrin, 11
November
1909. The
importance
of
this
problem
for Ein-
stein is confirmed
by a
letter
that
Einstein wrote
to
Ludwig
Hopf
before 12
January 1911, em-
phasizing
the
significance
of
his
equation
for the
coefficient
of
viscosity,
"because
from
viscosity
one can
learn
something
about the volume
of
dissolved
molecules"
("weil
man aus
der Vis-
kosität
etwas
erfahren kann über das Volumen
gelöster Moleküle").
[22]
See Einstein
1905j (Doc. 15), p.
18.
[23]
Einstein
to
Jean
Perrin,
11
November
1909.
[24]
In his
Autobiographical Notes,
Einstein
stated that his work
on
statistical
mechanics,
which
preceded
his dissertation, aimed
at
find-
ing
"facts
...
that would
guarantee as
much
as
possible
the existence
of
atoms
of
definite finite
size"
("Tatsachen
...
welche die Existenz
von
Atomen
von
bestimmter endlicher
Grösse
mög-
lichst sicher stellten")
(Einstein 1979,
p.
44;
translation,
p. 45).
For further discussion
of
Ein-
stein's
interest in the
problem
of molecular
di-
mensions,
see
the editorial
notes,
"Einstein
on
the Foundations
of
Statistical
Physics,"
p.
46,
and
"Einstein
on
Brownian Motion,"
pp.
206-
222.
[25]
In addition to the method
published
in the
dissertation, other methods
for
the determina–