112 



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 

 used in electric batteries. When such a 

 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) 6 ) 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. % 



to the nature of the substance considered. In this 



connection see: Weimarn, P. P. von, Grundziige der 



Dispersoidcheme. 12 7 p. Dresden,, 1915. For a 



clear and very readable discussion of colloids in 



general, see- Ostwald, Wolfgang, Die Welt der 



vernachlassigten Dimensionen. x + 219 p. Dresden 



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



Pig. 67. — Pfeffer osmometer (z), introduction to the physics and chemistry of colloids. 



with closed mercury manometer. 4th ed. i7<; p. London, 1922. Other books on 



(After Pfeffer.) this subjec 1 are mentioned in the List of Books, 



p. xix. — Ed 

 1 Pfefier, W., Osmotische Untersuchungen. Leipzig 1877. 



i 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. 



