Permeability 
191 
Table XIV 
Permeability of Various Precipitation Membranes to Positive Ions 
bound (in most cases) to Halogens. (Data from Walden) 
Membrane 
Glue-tannic acid 
Ni 2 FeCy 6 
Ni 3 (CoCy 6 ) 2 
Co 3 (CoCy 6 ) 2 
Cd 3 (CoCy 6 ) 2 
Co 2 FeCy 6 
Cu 3 (CoCy 6 ) 2 
Zn 2 FeCy 6 
Cu 2 FeCy 6 
Permeable to 
Alkalies, Cu, Ba, Mg, 
Fe", Ni, Co, Hg", Pb, 
A 1 
Alkalies, Mg, Ba, As 
Alkalies, Ba, As, Sb 
Alkalies, Ba, Sb 
Alkalies, Ba, Sb 
Alkalies 
Alkalies 
Li, Na, K, Tl, NH 4 , 
C 2 H 5 NH 3 , (C 2 H 5 ) 2 NH, 
(slightly permeable) 
Li, Na, K, Tl, NH 4 
Impermeable to 
?Cd, Zn, Mn" 
Ni, Co, Cu, Zn, Cd, Mn 
Ni, Co, Cu, Zn, Cd, Mn 
Fe, Ni, Co, Cu, Zn, Cd 
Cd, Co, Ni, Zn, Cu, Mn 
Mg, Ca, Sr, Ba (slightly 
permeable), As, Co, Ni, 
Cu, Zn, Cd, Mn 
Fe, As, Ca, Ba, Zn, Cu, 
Co, Ni, Mn, Cd 
Ca, Mg, As, Zn, Cu, Mn, 
Co, Ni, Cd, (C 2 H 5 ) 3 NH 
Be, Mg, Cu, Sr, Ba, As 
bases the diffusion of halides is greater the greater the atomic weight 
of the base. 
The effect on permeability of the addition of neutral salts to 
acids varies according to the acid. In some cases such addition is 
without effect (for example with hydrochloric and trichloracetic 
acids), in other cases there results a small hindrance to diffusion (as 
with sulphuric acid), while in other cases the addition of salts has a 
very significant effect (as with tartaric, glyceric and acrylic acids). 
Tammann’s observations on the penetration of 17 different dyes, 
including salts of colour bases, sodium salts of sulphonic acids, and 
acids, through three precipitation membranes, namely, glue-tannic 
acid, zinc ferrocyanide and copper ferrocyanide, showed seven 
exceptions to the regularity of the general order of permeability. 
Thus of the 17 dyes examined, eleven passed through the glue-tannic 
acid membrane, seven through the zinc ferrocyanide membrane and 
five through the copper ferrocyanide membrane. Nevertheless, 
fuchsin chloride, for instance, was able to penetrate the glue-tannic 
acid membrane and the copper ferrocj/anide membrane, but not the 
zinc ferrocyanide membrane, although in general the zinc ferrocya¬ 
nide membrane is more permeable than the membrane of copper 
ferrocyanide. An inverse case is that of cotton blue, to which the 
membrane of zinc ferrocyanide is permeable, but which can penetrate 
neither the copper ferrocyanide membrane nor the glue-tannic acid 
membrane, which is, in general, the most permeable of the three 
membranes examined. 
