DOC.
50
327
We
obtain
A
=
0.8.10-4
cm
=
0.8
micron.
This
number
has
an
uncertainty
of
±
25%
due to
the
low
accuracy
with
which
N
is
known.
[13]
It is
of interest
to
compare
the
mean
proper
motion
of the
microscopic
particles
we
just calculated with
that of
dissolved molecules
or
ions.
For
an
undissociated dissolved
substance
whose
coefficient
of
diffusion is
known,
A
can
be
calculated
from equation (7a).
For
sugar
at
room
temperature
we
have
0
^
D
= 24»6Q«60.
From
this
we
get
from
equation
(7a)
for
7
=
1
[14]
A
=
27.6 micron.
From
the
number
N
and
the molecular
volume
of
solid
sugar
we can
conclude that
the diameter
of
a
molecule
of
sugar
is of the order
of
magnitude
of
a
thousandth
of
a
micron,
i.e.,
about
one
thousand times smaller than the
[15]
diameter of the
suspended
particle
considered before.
According
to equation
(8a),
we
can
therefore
expect
A
to be
about
1000
times
larger
for
sugar
than for the particle with
a
diameter of
1
micron.
As
we
have
now
seen,
this
is indeed
approximately correct.
For
ions,
A ["A"
added
by
the translator]
can
be
determined
from
their
[16]
migration
velocity
l
from equation
(8). l
equals
the
quantity
of electri-
city
in
coulombs
that
would
flow
through
1
cm2 per
second at
a
concentration
v
=
1
of the
ion
in
question
and at
a
potential
gradient
of
1
volt
per
centimeter. In this
imaginary process,
the
velocity
v
of
the
motion of
the
ions (in
centimeter/second)
is
obviously given
by
the
equation
l
=
v.96,000.
[17]
Further, since
1
volt contains
108 electromagnetic
units,
and
the
charge
of
a
(univalent)
ion equals
9,600/N
electromagnetic
units,
the force
k
exerted
on one
ion
in the
process
imagined
will
be
k
=
108.9,600
N
[18]
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