50 BIOLOGICAL CHEMISTRY 



separated by a membrane. In this case the behaviour of the 

 two liquids is complicated by the nature of the membrane. 

 If all the substances can pass through the membrane the 

 behaviour is the same as it would be without the membrane. 

 A difference in vapour pressure (concentration) on the two 

 sides of the membrane will produce a temporary osmotic 

 pressure which falls again when the materials are equally 

 distributed. 



If, on the other hand, a substance is present which will not 

 pass through the membrane a permanent osmotic pressure is 

 produced. 



The permeability of a membrane is determined firstly by 

 the solubility of a substance in it, secondly by the molecular 

 size of the substance, and thirdly it is said that the electrical 

 charge on the membrane is also important. If the substance 

 is not soluble in the membrane it cannot pass through ; thus 

 a rubber membrane is permeable to ether but not to water. 

 The molecular weight also has some influence as the substances 

 with high molecular weight do not readily pass through 

 membranes. 



The influence of the electrical charge on the membrane is 

 probably unimportant. It is stated that a positively charged 

 membrane will repel positively charged ions. The negatively 

 charged ions will, however, be attracted and as they cannot 

 escape at the opposite side owing to the electrical attraction 

 between the positively charged membrane and negatively 

 charged ion, the charge on the membrane will be neutralised. 

 After the charge on the membrane is neutralised positive ions 

 will pass through as easily as negative ones, thus it follows 

 that the charge on the membrane cannot control the passage 

 of materials through it. 



COLLOIDS 



The substances which do not pass through ordinary mem- 

 branes such as parchment or parchment paper have been 

 termed colloids because of their similarity to glue,* whilst 

 substances that do diffuse through membranes are called 

 crystalloids. 



There seems to be every stage between a liquid containing 

 visible particles in suspension and a true solution. The 

 colloidal solutions are those that present properties inter- 

 mediate between microscopic particles and true solution. A 



* T. Graham, Phil. Trans., 1861, p. 220. 



