DOC.
25
SOLVAY
DISCUSSION
REMARKS
509
note,
"Einstein
on
Critical
Opalescence,"
pp.
283-285),
and
experiments
on
the "atom
of
electricity."
Einstein's
first
discussion remark
refers to
Keesom's derivation of
a
formula for
light scattering by
critical
opalescence,
first
published
in
a
footnote
to
Kamerlingh
Onnes
and
Keesom
1908b,
pp.
621-622. This remark
does
not
appear
in
the
published
version of the
discussion,
probably
because Einstein
changed
his
opinion
on
the
significance
of Keesom's contribution
(see
Einstein to
W.
H.
Julius,
18
December
1911).
Einstein's second discussion remark
concerns
the
experimental
evidence for the existence of
a
natural unit of
electric charge.
In
contrast to
Millikan's
results,
which seemed
to
demonstrate
conclusively
the
existence
of such
a
unit
Ehrenhaft's
experiments
on
ultramicroscopic
silver
particles suggested
that
there
is no
lower limit
on
electric
charge.
(See
Holton
1978
for
a
historical
discussion.)
How
to
explain
Ehrenhaft's
apparent
"subelectronic"
charges
constituted
a puzzle.
In
his
re-
mark,
Einstein claims
that
the
puzzle
was
solved
by
his
colleague
Edmund Weiß
in
Prague.
Weiß found
that
contrary
to
Ehrenhaft's
claim,
Stokes's
law
does
not
apply
to
these small silver
particles
(see Weiß
1911,
p. 631).
Weiß evaluated the
coefficient
of
mobility
for
each individual
particle
in his
experiments,
and found
that
this
coefficient
differed
from
particle
to
particle.
His
conclusion,
repeated
here
by
Einstein,
was
that
Ehrenhaft's
charge
determinations
were
not valid.
For Einstein's role in Weiß's
expe-
riments,
see
Einstein
to
Heinrich
Zangger, 7 April
1911.
Einstein had communicated
Weiß's results
to Perrin,
who referred
to
them
in his
report
(see
Perrin
1912,
p. 234).
Millikan later discussed Weiß's
experiments
and
emphasized
that
in
the
sequel
of
the
experiments by
Weiß and
Przibram,
the
scientific
world "ceased
to
concern
itself
with
the idea of
a
sub-electron"
(see
Millikan
1917,
p.
163,
and also
p. 153).
No.
137
(Perrin et
al.
1914, p. 206;
Perrin
et al.
1912,
p. 251).
The
first
remark does
not
appear
in
the German
or
French
printed versions, possibly indicating
Einstein's
intervention.
9)
Es ist
zu
bemerken,
dass
die
Opaleszenzformel
für
homogene
Substan-
zen
zuerst
von
Herrn Keesom
abgeleitet
ist,
und
zwar
auf
sehr
hübsche
Weise.
Ferner möchte
ich
daran
erinnern,
dass Herr
Weiss
in
Prag
zeigen
konnte,
warum
Ehrenhaft
zu so
kleinen Werten
von
e
geführt
wurde. Er bestimmte
bei
Silberteilchen in Luft deren
Beweglichkeit
aus
deren Brownscher
Bewegung
und
aus
ihrer
Geschwindigkeit
im elektrischen Felde
deren
Ladung
die
be-
friedigende
Übereinstimmung
mit den
übrigen Bestimmungen
für
e
zeigt.
Es
zeigte sich,
dass zwischen
Fallgeschwindigkeit
im Schwerefeld
und
Beweg-
lichkeit
verschiedener
Teilchen
kein
Zusammenhang
besteht,
woraus folgt,
dass die Teilchen sehr
unregelmässig gestaltet
sein
müssen. Ehrenhafts
Be-
stimmungen
von e
sind also
deswegen
illusorisch, weil
es
nicht
angeht
aus
der
Fallgeschwindigkeit
auf
die
Masse der Teilchen
zu
schliessen.