136 PROTOPLASMIC ACTION AND NERVOUS ACTION 



stances which were held back in filtration experiments; 

 for example, disks of 10 per cent content prevented the 

 passage of nearly all the colloids employed. From such 

 facts we should expect that a still higher degree of 

 impermeability (to crystalloids) would require a higher 

 degree of structural density; i.e., lower water-content. 



Experience with artificial semi-permeable membranes 

 of copper ferrocyanide bears out this expectation; this 

 is well seen in Morse's studies of these membranes, 

 described in his book, Aqueous Solutions.^ He found 

 that for the formation of good semi-permeable membranes 

 an extremely fine porosity in the supporting porcelain 

 cells was necessary; '^ excessive fineness of texture is 

 absolutely indispensable to the correct measurement of 

 osmotic pressure" (p. 15). A good semi-permeable 

 membrane is thus essentially a fine-textured structure of 

 water-insoluble material; hence it resists the passage 

 of water as well as of dissolved substances. Morse also 

 considers that the colloidal character of the precipitate 

 is an important factor in semi-permeabihty. Even 

 under the best conditions a high degree of semi-permea- 

 bility is difficult to obtain with precipitation-membranes, 

 and this property is subject to change; it is affected by 

 temperature (cf. pp. 85-86) and especially by electrolytes 

 (cf. pp. 91-92) which cause rapid deterioration in the 

 membrane. LiCl was the least harmful of the electro- 

 lytes investigated (cf. p. 214) and Morse gives determina- 

 tions of the osmotic pressure of this salt. 



Since fineness of texture is favorable to semi-permea- 

 bihty (i.e., to the production of osmotic effects), one 

 might expect that any insoluble porous partition would 



^ Morse, Aqueous Solutions, Carnegie Institute Publications (1914). 



