DISSERTATION
ON MOLECULAR DIMENSIONS 179
tron
[charge]
this would be
a simple
matter indeed.
I
would have done it
myself
but lack the reference material and the
time;
you
could also
bring
in
diffusion in
order
to obtain information about neutral salt molecules
in
solution.
Hast Du die absolute Größe
der
Ionen schon
ausgerechnet
unter
der Vorausset-
zung,
daß
dieselben
Kugeln
und
so
groß
sind,
daß die
Gleichungen
der
Hydro-
dynamik
reibender
Flüssigkeiten
anwendbar sind.
Bei
unserer
Kenntnis
der ab-
soluten Größe des Elektrons wäre dies
ja
eine einfache Sache.
Ich
hätte
es
selbst
gethan,
aber
es
fehlt mir
an
Litteratur und
Zeit;
auch die Diffusion könntest Du
heranziehen,
um
über die neutralen Salzmoleküle in
Lösung
Aufschluss
zu er-
halten.[73]
This
passage
is
remarkable,
because
both
key
elements
of
Einstein's method for
the
determination
of
molecular
dimensions,
the theories
of
hydrodynamics
and diffusion,
are
already
mentioned,
although
the reference to
hydrodynamics probably
covers
only
Stokes's law. While
a program very
similar to the first
of Einstein's
proposals
to Besso
is
pursued
in
Bousfield
1905a,
1905b,[74]
Einstein's
dissertation
can
be
seen
to be
an
elabo-
ration
of
the second
proposal, regarding
diffusion and neutral salt molecules. Einstein
may
thus have been
proceeding similarly
to
Nernst,
who first
developed
his
theory
of
diffusion
for the
simpler
case
of
nonelectrolytes.[75]
The
study
of
sugar
solutions could draw
upon
extensive
and
relatively precise
numerical data
on
viscosity
and the
coefficient
of
diffu-
sion,[76]
avoiding problems
of
dissociation and electrical interactions.[77]
V
The results obtained with
Einstein's
method for the determination
of
molecular dimensions
differed from those obtained
by
other methods at the
time,
even
when
new
data taken from
Landolt and
Börnstein 1905
were
used to recalculate them. In his
papers on
Brownian
motion,
Einstein cited
either
the value he obtained for
Avogadro's number,
or a
more
standard
one.[78]
Only
once,
in Einstein 1908c
(Doc.
50),
did he comment
on
the
uncer-
tainty
in the determination
of
this
number.[79]
By
1909
Perrin's
careful measurements
of
Brownian motion
produced a new
value for
Avogadro's number,
significantly
different
[73]
See Einstein
to
Michele
Besso, 17
March
1903.
[74]
For
a
critical
evaluation
of
Bousfield's
work, see
Dhar
1914,
p.
64.
[75]
See
Nernst
1888.
[76]
The tables
of
data for
sugar
solutions in
Landolt and Börnstein
1894 and 1905
are ex-
tremely
detailed.
[77]
For
a
discussion
of
these
problems,
see
Sutherland
1902,
pp.
167ff, and,
for
a
later
re-
view,
Dhar
1914; for
an
account
of
the
problem
of
internal friction in
electrolytes, see
also Herz-
feld
1921,
pp.
1013-1018.
[78]
In Einstein 1905k
(Doc. 16),
Einstein
cited
a
value for Avogadro's number taken from
the kinetic
theory
of
gases;
in Einstein 1907c
(Doc.
40), he cited the value obtained in Ein-
stein 1906c
(Doc. 33);
in Einstein 1908c
(Doc.
50),
he
again
cited
a
value close to that derived
from
gas theory.
[79]
See Einstein 1908c
(Doc. 50),
p.
237,
fn.
2.