394
DOC.
25
SOLVAY DISCUSSION REMARKS
VI.
PERRIN
Perrin's lecture
(Perrin 1912)
is
an
exhaustive review
of
the
experimental
evidence in favor of
the
existence of
atoms.
Although
much of it
concerns
the work of his
own group on
Brownian motion and related
matters,
Perrin also mentioned studies of critical
opalescence by
Smoluchowski, Keesom,
and Einstein
(see
the editorial
note,
"Einstein
on
Critical
Opalescence"),
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
remark,
Einstein claims
that the
puzzle was
solved
by
his
colleague
Edmund Weiss
in
Prague.
Weiss found
that
contrary
to
Ehrenhaft's
claims,
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
experiments,
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.
251).
The
first remark does
not appear
in the German
or
in
the
French
printed
versions,
presumably indicating
Einstein's intervention.
9)
It should be noted that
Mr.
Keesom
was
the first to
derive
the
opalescence
formula for
homogeneous substances,
something
which he did in
a very
elegant
way.
Further,
I would like to
remind
you
that
Mr. Weiss
from
Prague
was
able to show
why
Ehrenhaft had been led
to
such small values
of
E.
He
investigated
silver
particles
in
the
air,
and determined their
motility
from their
Brownian
motion and their
charge
from their
velocity
in
the electrical
field,
which showed
good
agreement
with
the other
determinations of
e.
It turned
out
that
no
connection
exists
between the
velocity
of
fall
in
the
gravitational
field and
the
motility
of
different
particles,
from which it follows
that
the
particles
must
be of
very
irregular shape.
Ehrenhaft's
determinations of
e
are
thus
illusory,
because it
will not do to draw conclusions
about the
mass
of the
particles
from the
velocity
of
fall.
VII.
NERNST
Einstein's first
comment
on
Nernst's lecture
(Nernst 1914)
refers
to
the
difficulties
of
generalizing
the
quantum
hypothesis
to
more
than
one
dimension. Einstein confronted
an objection
raised
by
Lorentz
against
Nernst's
decomposition
of
a
classical three-dimensional
oscillation
into
three
circular
components.
Nernst
not
only
used
this
decomposition
to
infer
the
equality
of
kinetic and
potential energy
for each
of the
circular
components,
but
also
to
attempt
to
make
the
different roles of kinetic and
potential energy
in his
understanding
of the