270 
MR. J. LARl^IOR ON A OYNA^IICAL THEORY OF 
to he negligible ; the eftective osmotic pressure then diminishes, being accompanied 
]jy diffusion in the pores which involves dissipation of energy that would otlieiwise 
produce osmotic effect. Thus the proposition that the osmotic pressure between two 
fluids is equal to the free or available energy of mixture per unit volume of transpi¬ 
ration, gives only the limiting value which applies to partitions with 23ores sufficiently 
narrow. In the equilibrium stage of transpiration through a colloid membiane, 
operating hy absorption into one face of the membrane and evaporation from the 
other, the limiting pressure may however be reached, provided the action does not 
involve irreversible thermal processes in the membrane. 
The osmotic pressure between a solution and the pure solvent is, from another 
point of view, the mean aggregate of the forces that have to be applied to the 
individual molecules of tlie dissolved substance in order to prevent them from travel¬ 
ling across the interface into the pure solvent, whether that forcive be applied by the 
resistance of a material partition, or as in the ca.se of ions diffusing across the interface 
between two salt solutions in contact, by the pull of the electric field which the diffu¬ 
sion has produced—the unmodified molecules of the solvent being in each case free to 
move either way. Viewed in this light, there is nothing occult or merely analogical 
—unless it be the presence of ions—in the principles by which Neenst determines 
the constitution of the layer of transition which gives rise to the potential diflerence 
between two salt solutions, and so determines the voltaic and thermoelectric differences 
of potential at such transitions, by balancing a bodily force arising from osmotic 
pressure by another arising from the electric field due to the reacting double layer 
generated by the dlftusion. 
56. Suppose that the pressures on the two sides of a porous partition separating 
dielectric fluids are adjusted so that there is no flow across it. When an electric 
field is introduced this equilibrium is destroyed by the effective electric tractions on 
the interfaces of separation between the dielectric fluids in the individual pores. To 
re-establish equilibrium a diflerence of pressure at the two sides of the interface, 
equal to that of the electric tractions (§ 37), must be called into play : that is, an 
electric field infliiences the value of the osmotic pressure between dielectric fluids. 
This effect is of course directly connected, through a cyclic process, with an influence 
on vapour tension (§81, infra). Its amount is to a great extent independent of the 
size of the pores; though when the pores are of molecular dimensions it mainly arises 
from a bodily forcive on the contained filaments of fluid. This electric osmotic 
pressure wdll then even hold good wdth respect to liquids which readily mix ; for the 
obliteration of the sharpness of tlie interface in the narrow^ tubes or pores of the 
partition will take place very slowly, while the formulcTe of this memoir for electric 
tractions are precisely those which hold good for a gradual transition. 
This action is different from the one discovered by Quincke and discussed by 
VON Helmholtz,'” forming in fact a further extension of the scope of the principle of 
* TON Helmholtz, “ St-ndien iiber clectrische Gvenzscliichten,” ‘tYied. Ann.,’ 7, 1879. 
