PHYSIOLOGY OF NUTRITION 



Membranes of animal bladder, parchment paper and collodion, as well as 

 the so-called precipitation-membranes, are all used for osmotic experiments. 

 Cellulose membranes, giving the cellulose reaction with zinc chloride and iodine 

 (Baranetskii, 1870) can be produced by treatment of collodion membranes 

 with ferric chloride. Of the above-mentioned membranes, animal bladder is 



much like the plant cell wall in its osmotic 

 \ properties, while precipitation membranes 



are only very slightly permeable to many 

 substances and can give rise to high osmotic 

 pressures. Suitable supports must be pro- 

 vided for these delicate membranes. Pfeffer 1 

 employed porous clay cylinders such as are 

 Ж1 used in electric batteries. When such a 



f porous cell is filled with a copper sulphate 



(CuS0 4 ) solution and placed in a solution 

 of potassium ferrocyanide (K 4 Fe(CN)6), a 

 membrane of copper ferrocyanide (Cu 2 Fe- 

 (CN)e) is precipitated in the porous wall. 

 Similar precipitation membranes may be ob- 

 tained with other substances, such as iron 

 silicate. To measure osmotic pressure the 

 porous cylinder, with its membrane, is filled 

 with the solution to be studied and is con- 

 nected with a mercury manometer, the 

 cylinder being submerged in water (Fig. 67). 

 The magnitude of the pressure exerted at 

 equilibrium is then read upon the manometer.* 



Fig. 67. — Pfeffer osmometer (z), 

 with closed mercury manometer. 

 (After Pfeffer.) 



to the nature of the substance considered. In this 

 connection see : Weimara, P. P. von, Grundzüge der 

 Dispersoidcheme. 127 p. Dresden, 1915. For a 

 clear and very readable discussion of colloids in 

 general, see- Ostwald, Wolfgang, Die Welt der 

 vernachlässigten Dimensionen, x+219 p. Dresden 

 and Leipzig 191 5. Also see: Hatschek, Emil. An 

 introduction to the physics and chemistry of colloids. 

 4th ed. 17 j p. London, 1922. Other books on 

 this subject are mentioned in the List of Books, 

 p. xix. — Ed 

 1 Pfeffer, W., Osmotische Untersuchungen. Leipzig 1877- 



* The most perfect precipitation membranes yet made are those of Morse and his 

 coworkers, who have been engaged for many years in very thorough studies on the osmotic 

 pressures developed by concentrated solutions. This work has been carried out in the 

 Chemical Laboratory of the Johns Hopkins University. Much improved forms of the 

 Pfeffer cell have been employed and the copper ferrocyanide membranes of these writers 

 have proved quite impermeable to cane sugar for many days, even with very high 

 pressures. For accounts of this work see: Morse, H. N., and Horn, D. W., The 

 preparation of osmotic membranes by electrolysis. Amer. chem. jour. 26: 80-86. 1901. 



