DOC.
50
319
of the
solution
is
higher
in
A
than in
B,
then
an
external
force,
directed
to
the
left,
must be
applied
to
the piston
to
maintain it in equilibrium,
and
this force is
equal to
the difference
between
the
two
osmotic
pressures
exerted
by
the dissolved substance
on
the
piston from
the left
and from the
right, respectively.
If this external
pressure
is
not applied to
the piston,
the latter
will
move
to
the
right under
the influence of
the
greater
osmotic
pressure
exerted
by
the solution
in
A
until the concentrations
in
A
and
B
K
Fig.
93
no
longer
differ. This consideration demonstrates that it is
precisely
the
forces of osmotic
pressure
which
cause
the equalization
of
concentrations in
diffusion; because
we
can
prevent
diffusion, i.e., the
equalization of
concen-
trations,
by
counterbalancing
the osmotic differences,
which correspond to
differences in concentrations, with external
forces
acting
on
semipermeable
walls. It
has been
known
for
a long
time that osmotic
pressure can
be
considered
as
the
motive
force in
processes
of diffusion.
As
we
know,
Nernst
used
this
as
the basis for his
investigation
on
the connection
between ionic
mobility,
the coefficient
of
diffusion,
and
the
EMF
in concentration cells.
Let
diffusion take
place along
the cylinder's axis inside the
cylinder
Z (Fig.
94),
whose
cross
section shall
be
=
1.
Let
us
first
examine
the
osmotic forces
causing
the
diffusional
motion
of the dissolved substance
contained
between
the
infinitesimally
close
planes
E
and
E'.
From
the
left, the force of osmotic
pressure
p
acts
on
the
lamina's
boundary
surface
E,
and from
the right, the
pressure
p' acts
on
the
boundary
surface E';
the
resultant
of the
pressure
forces is therefore
[4]
p
-
p'.
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