192 
Walter Stiles 
It is to be noted in regard to the case oi the permeability of the 
copper ferrocyanide membrane to fuchsin chloride, that according 
to Meerburg (1893) the dye is only able to penetrate the membrane 
until the latter is completely impregnated with dye. 
Membranes then differ among themselves in regard to their 
permeability, and the permeability of any particular membrane is 
different to different substances. Generally the order of permeability 
to different substances is the same in the case of different membranes 
but there are exceptions to this rule. It would appear that mem¬ 
branes are much more permeable to halides than to sulphates and 
also considerably more permeable to salts of monovalent metals than 
to those of divalent metals. In some cases the permeability depends 
on the degree of dissociation of the substance but this is not by any 
means a universal rule. 
Ostwald (1890) pointed out that in the case of a dissociated salt 
the membrane need only be impermeable to one ion of a salt in 
order to prevent both ions from passing, for on account of the 
electrostatic attraction of the oppositely charged ions the permeable 
ion will only travel to such a distance that its tendency to diffuse 
balances the electrostatic attraction. 
It will be observed that very little quantitative work has been 
done on the permeability of membranes, and consequently exact 
data as to the influence of temperature and other factors on per¬ 
meability are for the most part wanting. The influence of pressure 
on the passage of water through membranes has however been the 
subject of investigation by several workers, e.g. Schmidt (1856), 
Pfeffer (1877), Sebor (1904), Bigelow (1907) and Bartell (1911), from 
whose observations, and especially from those of Bigelow and 
Bartell, it appears that the rate of passage of water through mem¬ 
branes is proportional to the pressure. The membranes investigated by 
these different authors include various animal membranes (Schmidt), 
membranes of collodion and parchment paper (Bigelow), porcelain 
(Bigelow, Bartell), and copper ferrocj^anide (Pfeffer, Sebor, Bartell). 
The influence of temperature on the permeability of membranes 
of collodion to water at constant pressure was investigated by 
Bigelow and his results extended to porcelain by Bartell. The 
conclusion of these investigators is that the permeability expressed as 
the quantity of water passing through unit area of membrane in unit 
time, is given by the relation M e = M 0 (1 -f* 0-0336 80 + 0-0002210 2 ), 
where M e , M 0 are the permeabilities of the membrane at temperatures 
0 °, o° respectively. As this formula is that connecting the viscosity 
of liquids at different temperatures the authors conclude that the 
