I 12 



PRINCIPLES OF GENERAL PHYSIOLOGY 



a membrane of water, supported in some way, as in wet parchment paper. This 

 allows carbon dioxide to pass through, but keeps back oxygen and nitrogen. 

 Consideration will show, however, that, since these latter gases are not absolutely 

 insoluble in water, after a sufficiently long time there will be no difference in 

 composition between the gaseous mixture on the two sides of the membrane. A 

 membrane of palladium, as investigated by Ramsay (1894), is permeable to 

 hydrogen, but not to oxygen, either because the hydrogen is soluble in it, or because 

 a reversible compound of some kind is formed, which dissociates under a lower 



tension of hydrogen. 



As regards membranes like 

 parchment paper, gelatine, col- 

 lodion, etc., which allow water 

 t'ind crystalloids to pass, but hold 

 back colloids, it is practically 

 certain that they have a porous 

 structure. Many facts point to 

 this. Biltz (1910) showed that 

 the rate of passage of dyes through 

 parchment paper is in direct rela- 

 tion to their molecular dimensions. 

 Heymans (1912) found that certain 

 micro organisms were able to pass 

 through this paper. When speak- 

 ing of the ultra-filter of Bechhold, 

 I stated that the permeability 

 could be varied by taking different 

 strengths of collodion, and Bech- 

 hold himself (1908) has determined 

 the dimensions of the pores of 

 various membranes by pressing air 

 through them, when covered with 

 water. Schoep also (1911) has 

 been able to control the dimensions 

 of the pores by mixing castor oil 

 and glycerol with the collodion 

 used to prepare the membranes. 



The copper ferrocyanide mem- 

 brane has played a great part in 

 the investigation of osmotic pres- 

 sure ; the discoverer of it will be 

 of sufficient interest to the reader 

 to warrant the introduction of his 

 portrait (Fig. 44). 



When a solution of potassium ferro- 

 cyanide comes into contact with one of 



copper sulphate, a membrane in the form of a colloidal gel of copper ferrocyanide is formed at 

 the surface of contact. This gel contains a considerable percentage of water ; if allowed to 

 dry, it becomes impermeable altogether, even to water. In order to be able to perform 

 experiments with such a membrane, it must be supported by being formed in the pores 

 of a cylinder of unglazed porcelain, or, in some cases, in collodion. Further details will 

 be found in the chapter on osmotic pressure. This membrane, although colloidal, obviously 

 has interspaces I >rt \vft-u its constituent elements of much smaller dimensions than those 

 of gelatine or collodion, since, as Traube showed (1867), it does not allow i-anc-sugar to 

 pass through, nor even many salts. Its discoverer regarded it as a " molecular sieve," 

 in that its pores, while large enough to allow water to pass through, were too small to admit 

 dissolved substances. Closer investigation, however, showed that there are some salts which 

 can pass through. Thus, potassium chloride was found to do so, while barium chloride, 

 calcium chloride, potassium sulphate, barium nitrate, and ammonium sulphate could not. 



It is pointed out by Ostwald (1890) that it is not necessary to assume that 

 a membrane is impermeable to both ions of a salt, when it is found that the 

 salt in question is not allowed to pass. If one ion only is allowed passage 



44. PORTRAIT OF MORITZ TRACBK. 



