PERMEABILITY OF STATIC MEMBRANES 187 



to compare the surface energy at points on the circumference of a circle 

 at whose center the spreading substance touches the liquid. Substances 

 like myristic acid are found to spread in two stages. In the first stage 

 the surface is covered by a unimolecular " expanded " film under no 

 compression. The second stage follows when the expanded film becomes 

 more closely packed and still maintains its unimolecular thickness. 



The ring method was used by Cary and Rideal [1925] to follow surface- 

 energy changes as illustrated by the progressive film formation of a 

 crystal of a fatty acid, as, for instance, myristic acid, when it was 

 brought into contact with the surface of a 0.01 N solution of hydrochloric 

 acid in water. The velocity of spreading on water was found to be of the 

 order of 20 cm/sec. 



Water, having a high cohesion, does not spread on organic liquids. 

 Organic liquids of moderately high cohesion spread on water. Any 

 diminution of the surface energy of the water, by an adsorbed film, 

 diminishes the tendency of the upper liquid to spread. 



The property of castor oil to spread rapidly as a thin film would seem 

 to help explain the rapidity and thoroughness with which it coats the 

 intestinal walls when it is used as a purgative. It has also been demon- 

 strated that castor oil is a better lubricant for machinery than mineral 

 oil of equal viscosity. 



Permeability of Static Membranes 



It has been found possible to construct partitions which, when used to 

 separate pure water from a solution of crystalloids like sugar or salt, 

 will allow the water to diffuse through them, but not the dissolved 

 crystalloids. A film of colloid (glue-like) material such as starch coated 

 on a supporting sheet of porous paper, if used as a partition between pure 

 water and a solution of crystalloids and colloids, will allow the crystal- 

 loids to pass through and diffuse into the water, but it will entirely block 

 the passage of the colloids. If the partition is a structure which allows 

 only the solvents to pass through, it is designated as semi-permeable. 



Some animal bladders and parchment sheets will allow water and 

 dissolved crystalloids to pass through but will prevent the passage of 

 colloidal substances. Animal and vegetable membranes that have the 

 property of allowing the solvent to pass through them and of preventing 

 some or all of the solute from passing are also considered semi-permeable. 



The botanist de Vries [1888] was one of the first to attribute the shrink- 

 age of plant cells, placed in dilute sugar solutions, to the semi-permea- 

 bility of the cell membrane. He observed, with the aid of a microscope, 

 that plant cells had comparatively rigid cellulose walls enclosing a mem- 

 brane filled with protoplasm. Reproductions of his original osmotic 



