DIFFUSION AND OSMOSIS 45 



water passed into the bladder more rapidly than the alcohol 

 passed out ; the bladder became distended, the internal pressure 

 increased, and the liquid spirted out when the bladder was 

 pricked by a pin. This passage of certain substances in 

 solution through an animal membrane is called Osmosis, and 

 membranes which exhibit this property are called osmotic 

 membranes. 



Precipitated Membranes. — In 1867, Traube of Breslau dis- 

 covered that osmotic membranes could be made artificially. 

 Certain chemical precipitates such as copper ferrocyanide can 

 form membranes having properties analogous to those of 

 osmotic membranes. With these precipitated membranes 

 Traube made a number of interesting experiments. These 

 have lately been collected in the volume of his memoirs 

 published by his son. 



Osmotic Membranes. — Osmotic membranes were formerly 

 called semi-permeable membranes, being regarded as membranes 

 which allow water to pass through them, but arrest the passage 

 of the solute. This definition is inexact, since no membrane 

 permeable to water is absolutely impermeable to the solutes. 

 All we can say is that certain membranes are more permeable 

 to water than to the substances in solution, and are moreover 

 very unequally permeable to the various substances in solution. 

 As a rule a membrane is much more permeable to a solute 

 whose molecule is of small dimensions. Molecules of salt, for 

 instance, pass through such a membrane much more quickly 

 than do those of sugar. The term " osmotic membrane " 

 should therefore in all cases replace that of " semi-permeable 

 membrane." 



Osmotic membranes behave exactly like colloids. The 

 resistance which they oppose to the passage of different 

 substances varies with the nature of the liquid or solute 

 concerned. There is no real difference between the passage 

 of a solution through an osmotic membrane and its diffusion 

 through a colloid. The protoplasm of a living organism, 

 being a colloid, acts exactly like an osmotic membrane so 

 far as regards the distribution of solutions and substances in 

 solution. 



