138 



PHYSIOLOGY. 



B 



sure measure that way is called the osmotic pressure of the solution, 

 from the Greek to force through. Osmosis is the passage of a stream 

 of water-molecules through a membrane. 



Osmotic Pressure. 1 



Saw a Pasteur-Chamberland filter in half. The cylinder is then 

 dipped in dilute hydrochloric acid, which is sucked through the wall 

 of the cylinder by a hydraulic airpump in order to remove any caolin 

 dust that might choke its pores; then rinse with 

 water in a similar way. A beaker is now filled 

 with a solution of potassium ferrocyanide (139 

 grams per liter), the cylinder is dipped into it, and 

 the solution is sucked through its wall. After the 

 cylinder has been again rinsed in water, it is dipped 

 into a second beaker containing a copper solution 

 (249 grams of the salt per liter), the inside of the 

 cylinder being also filled with the solution. A layer 

 of the cylinder, and this precipitate constitutes the 

 semipermeable precipitation membrane which is 

 permeable for water, but impermeable for salts. 



If we introduce a sugar solution into cell C 

 prepared in this manner and close it with the 

 stopper of rubber S, which is perforated by the 

 tube AB, then when C is dipped into pure water, 

 the sugar endeavors to pass from the place of 

 higher concentration (the solution) to that of 

 lower concentration (the water without tlue cell). 

 But this movement is opposed by the semiperme- 

 a ^^ e membrane, and in consequence the sugar 

 exerts a pressure upon the membrane. Since this 

 wall, however, is unyielding and so resists the 

 pressure, a pull is exerted upon the water by the solution which tends to 

 dilute the latter. This comes to pass when the solution enters the tube 

 and the water from G streams through the membrane into the cell 

 and dilutes the solution. This process goes on until the resulting 

 hydrostatic pressure in AB prevents the further entrance of the 

 water. When equilibrium has been established this hydrostatic 

 pressure is equal to the osmotic pressure of the solution. Con- 

 versely, however, the latter may be measured by ascertaining the 



3^ _ Osmometer 

 (COHEN.) 



literature consulted: Cohen's "Physical Chemistry," 1903. 



