METHODS OF ELECTRICAL STIMULATION 19 



the artificial membrane is great ; the osmosis of 

 salts through living membranes may be very rapid. 

 When the liquid in the tube is high enough to 

 balance the force with which the water would 

 pass through the membrane, the flow ceases. 

 The difference in water level is the osmotic 

 pressure, the analogue of gas pressure. The maxi- 

 mum osmotic pressure is so great that only perfect 

 semi-permeable membranes will support it. 



It will be obvious upon reflection that osmotic 

 pressure must be independent of the semi-per- 

 meable membrane. The solvent is continuous 

 through the membrane. The only function of 

 the membrane is to render the osmotic pressure 

 visible. 



With constant temperature the osmotic pres- 

 sure is nearly proportional to the concentration 

 of the solution. 



Concentration Osmotic pressure 



of solution of in centimetres Ratio. 



cane-sugar. of mercury. 

 Per cent. 



1 53.5 53.5 



2 101.6 50.8 

 2.74 151.8 55.4 

 4 208.2 52.1 

 6 307.5 51.3 



The osmotic pressure increases as the tempera- 

 ture rises. 



