Permeability 189 
copper ferrocyanide membrane to a number of salts of the alkali and 
alkaline earth metals are collected in the following tables. 
Table XI 
Permeability of Precipitation Membranes of Copper Ferrocyanide to 
a Number of Sulphates. (Data from Tammann) 
Salt 
(NH 4 ) 2 S 0 4 
k 2 so 4 I 
Na 2 S0 4 ( 
Li 2 S 0 4 
MgSO, 
Quantity passing through membrane 
small quantity 
smaller quantity 
a trace 
1 ABLE XII 
Permeability of Precipitation Membranes of Copper Ferrocj^anide to 
Chlorides, Bromides and Nitrates of Metals of Alkalies and Alkaline 
Earths. (An asterisk indicates that traces only pass through the 
membrane.) (Data from Tammann) 
Anion 
Ration Chloride Bromide Nitrate 
Ammonium 
Potassium 
Sodium 
Lithium 
Barium 
Strontium 
Calcium 
Magnesium 
I All penetrate the membrane in large 
V quantities; much more so than the 
corresponding sulphates. 
perm. perm.* perm.* 
perm.* perm.* imperm. 
imperm. imperm. imperm. 
imperm. imperm. imperm. 
From quantitative investigations on the diffusion of acids through 
the copper ferrocyanide membrane, Tammann came to the conclusion 
that the rate of passage of the acid through the membrane was 
chiefly dependent on the degree of dissociation of the acid, the more 
dissociated the acid the more rapid its passage through the mem¬ 
brane. 
An investigation by Walden (1892), the results of which were 
published almost contemporaneously with those of Tammann just 
quoted, agrees on the whole with these latter. Walden found, however, 
that membranes of copper ferrocyanide and zinc ferrocyanide had 
distinctly different permeabilities. Altogether the permeabilities of 
eleven precipitation membranes to a large number of acids and salts 
were examined. The membranes employed are recorded in the 
following table. All these membranes were impermeable to tannin, 
while all were permeable to the following anions when bound to 
alkali metal kations (K, Na, ?Li, NH 3 ): F, Cl, Br, I, CN, CNS, C 10 3 , 
C 10 4 , Br 0 3 , I 0 3 , N 0 2 , N 0 3 , formate, acetate, trichloracetate, iso- 
