PHYSICAL METHODS IN BLOOD EXAMINATION. 



249 



their impermeability for many salt or other solid molecules 

 may be shown. Some membranes are permeable for certain 

 salts but not for others. The fact of the existence of pressure 

 within such a membrane may be shown by the following well- 

 known experiment, in which a copper ferrocyanide sheath or 

 membrane is made in a different manner. 



Ex. Procure a small fine grained porous battery cell, about 3 to 4 inches 

 long and i inch in outside diameter, and clean it thoroughly. This may be 

 done by washing the cell with water, then with weak hydrochloric acid and 

 finally with w r ater very thoroughly. Close the 

 cell with a perforated rubber stopper, pass a 

 glass tube through the perforation and con- 

 nect the outer end of this with a suction pump. 

 On dipping the cell in water or the acid it 

 may be drawn through the pores of the cell 

 to effect the cleaning. When the cell is clean 

 it is placed in a potassium ferrocyanide solu- 

 tion containing about 150 gm. per liter and 

 solution drawn through by means of the pump 

 until the pores are thoroughly filled. Then 

 the cell is washed, inside and out, with dis- 

 tilled water and immersed in a blue vitriol 

 solution containing about 250 gm. per liter. A 

 precipitate is thus formed within the pores of 

 the cell, which is allowed to remain some 

 hours in the solution. The cell is then re- 

 moved, washed with water and is ready for 

 use. Fill it with a 5 per cent cane sugar solu- 

 tion, close with the rubber stopper and long 

 narrow glass tube and immerse the cell in a 

 beaker of distilled water the temperature of 

 which should be the same as that of the sugar 

 solution. After a short time liquid begins to 

 rise in the glass tube which serves as a 



kind of manometer. This is in consequence of the entrance of water to 

 the sugar solution. Sugar can not pass out in the other direction as the 

 precipitate membrane is not permeable for it, but it is readily permeable 

 for the water. The sugar in its effort to pass out to the water exerts a 

 pressure on the retaining membrane, and it is because of this pressure 

 that the water is able to enter the cell. The flow of the water continues 

 until its hydrostatic pressure exactly balances the sugar or osmotic pres- 

 sure. In some cases mercury manometers attached to such cells register 

 pressure of several atmospheres. 



The pressure actually observed in such an apparatus is just short of 

 that required to press the solvent, water, through the membrane in the 



FIG. 23. Apparatus for 

 observing and measuring 

 osmotic pressure. 



