THE PERMEABILITY OF MEMBRANES 113 



electrostatic attraction on the part of the oppositely charged ions will prevent 

 the permeable ion from travelling further than such a distance at which its 

 osmotic pressure balances the electrostatic force. Copper ferrocyanide is 

 permeable to both ions of potassium chloride; therefore, when it is found to 

 be impermeable to calcium chloride, it must be the calcium ion which is held 

 back. Similarly, in the case of potassium sulphate, it must be the SO 4 " ion 

 to which the membrane is impermeable. 



The fact that the membrane is not completely semi-permeable has led some 

 observers to hold that its permeability or otherwise is a matter of solubility in 

 the substance of the membrane itself. This view does not really lead us any 

 further, and, if we introduce the modern conception of the hydration of solutes, 

 and especially of their ions, it is still possible to look upon the membrane as a sieve. 

 Substances when dissolved become associated with a number of molecules of the 

 solvent, varying with the chemical nature of the solute. Thus, according to 

 J. C. Philip (1907), each molecule of potassium chloride has 7 to 11 molecules of 

 water associated with it, while copper chloride has about 21, and so on. Another 

 fact, which tends to support Traube's view, is that, as he found, a copper 

 ferrocyanide membrane, permeable to potassium chloride, becomes impermeable to 

 it when infiltrated with silver chloride (Traube, 1899, p. 261). It does not seem 

 likely that there should be any material difference between the solubility of 

 potassium chloride in silver chloride or in copper ferrocyanide ; if any, one would 

 expect it to be more soluble in the chloride, according to the old law, " similia 

 similibus solvuntur" (Rothmund, 1907, p. 112). On the other hand, it is to be 

 presumed that any pores present would be narrowed by deposition of silver chloride 

 on their walls. 



A detailed investigation of the permeability of a large number of precipitation 

 membranes was undertaken by Paul Walden (1892). If the table on pp. 716 and 

 717 of his paper be consulted, various facts will be noted which have a bearing 

 on the question before us. The membranes can be arranged in order of merit, as 

 regards impermeability to the substances tested. Tannin-gelatine is the lowest in 

 the series, being permeable to all -except tannin itself ; while copper ferrocyanide 

 is the highest, being impermeable to a larger number than any of the others. A 

 significant fact is that none of the membranes comes out of its place as regards 

 any particular substance. That is, assuming that the pores increase regularly in 

 dimensions from the copper ferrocyanide to the tannin-gelatine, no substance is 

 found which diffuses through a membrane having the smaller pores while being 

 held back by that with the larger pores, as might happen on the solution theory. 

 The behaviour of the hydrochlorides of the three ethylamines is of interest. The 

 copper ferrocyanide membrane is readily permeable to that of monoethylamine, 

 slightly permeable to that of the diethylamine, impermeable to that of the 

 triethylamine, following the increase of molecular dimensions. 



The difficulty frequently arises, however, as to the proof that the membrane is not 

 chemically acted upon, or injured in its integrity, when it appears to be permeable to a 

 particular solute. This consideration seems to deprive Tammann's experiments with dyes 

 (1892, p. 257) of much of their value, although this observer draws the conclusion that there 

 are dyes which pass a membrane which is supposed to have the smaller pores, while bein,g 

 held back by one with the larger pores, and that Traube's theory does not hold. In Walden's 

 experiments, the permeability of the membranes composed of the ferrocyanides of zinc and of 

 copper is identical, whereas in those of Tammann the zinc membrane shows itself to be 

 permeable to dyes to which the copper one is not ; it is even stated to be permeable to 

 " Baumwollenblau " to which the tannin-gelatine membrane is impermeable, and even parch- 

 ment paper only slightly so. If we neglect quantitative differences, which are very difficult 

 to judge satisfactorily, there are only two out of Tammann's seventeen dyes which fall out 

 of line. " Baumwollenblau " is one of these and the other is fuchsin-chloride, to which copper 

 ferrocyanide is permeable, zinc ferrocyanide not. According to Cain and Thorpe (" Synthetic 

 Dye-stuffs," 1913) "cotton-blue" is a mixture of ammonium and sodium salts of di- and tri- 

 sulphonic acids of rosaniline blue. Since even parchment paper is impermeable to the salt of the 

 mono-sulphonic acid ("aniline-blue ") it is difficult to believe that a zinc ferrocyanide membrane 

 (if perfect) should be permeable to the " cotton -blue " mixture. The experiments of Biltz 

 (1910, p. 117) on the passage of dyes through parchment paper, have been referred to above. 

 These experiments show an unmistakable relation between the molecular dimensions of the 

 dye and its ability to pass through the paper. If the number of atoms is less than 45, it 

 passes through quickly ; above 45, slowing begins to show itself ; between 55 and 70, the 



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