THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
271 
electrolytic dissociation, in which a stream of conducting fluid forced through a 
porous non-conducting partition produces an electric current across it, and conversely 
an electric current forced across the partition carries the fluid with it. Over the 
surface of each pore there is, on the present view, the intrinsic static potential 
difierence between the partition and the fluid, due to strong orientations of the polar 
molecules of these two media which lie along the interface, under their mutual 
influence which stands in place of von Helmholtz’s attraction of matter for elec¬ 
tricity as the exciting cause of voltaic phenomena;''' and this difierence will l)e in 
time diminished by the presence of free ions which become attached among the out- 
Avard-pointing poles, thus constituting a reverse potential difierence Avith AAdiicli 
electro-capillarity deals. The flow of fluid through the pores carries on some of these 
ions along Avith it, which thus constitute tlie observed electric current. Sudden 
diminution in the extent of the surface would act similarly by crushing them out, as 
in the observed electrification near AAmterfalls : rapid extension of the surface, as in 
the formation of drops in air, should conversely eliminate the effect of the ions bound 
to the polarized air-film on the surface, by spreading them over a Avider area, and so 
increase the potential difierence towards the limiting statical Amlue. On the other 
hand, Avhen the media in contact are very dilute electrolytic solutions in the same 
solvent, the calculations of Nernst shoAv that the potential difierence is Avholly an 
affair of ionic diffusion, as indeed it must be if the efficient polar molecules are all 
ionized; in that case the normal potential difierence Avill require a sensible time to 
become established. When, in the case of a mercury electrode dropping into an 
electrolytic solution, sufficient time is not alloAved, the part of the actual potential 
difference which arises from this cause and not from the intrinsic statical orientation 
of the molecules, Avill tend to a vanishing limit, except in so far as it is continually 
restored by a polarization current in the electrolyte. 
57. In the case of very dilute solutions it is possible to obtain a definite expression 
for the limiting, or maximum, osmotic pressure. After a certain stage of dilution, 
each dissolved molecule is efiectively out of touch Avith its fellows and is completely 
environed, by a collocation of molecules of the sohmnt ; further dilution therefore does 
not involve any sensible change in the mutual free energy of the solvent and the 
dissolved molecules ; all that occurs is a wider separation of the dissolved molecules 
in space, Avith such energy changes as may be directly concerned in it. Siq)pose noAv 
that the dissolved substance is a gas, and that the solution is separated from the pure 
solvent by a partition which the latter can traverse Avhile the gas cannot : Avhether 
such partitions are knoAvn to exist is inessential to the theoretical argument, the 
function of the partition being merely passiAm constraint exerted on the aggregoAe of 
* lHelmiioltz liaci to be conteut in bis analysis with the crude conception that different kinds of 
matter attract electricity differently. On a scheme like the present the obvious explanation is that 
the polar molecules of the two substances act on each other across the interface, producing a certain 
regularity of orientation which forms the intrinsic double layer to which the potential difference is due.] 
