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 

 (CUSO4) solution and placed in a solution 

 of potassium ferrocyanide (K 4 Fe(CN)e), 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: Weimarn, P. P. von, Grundziige der 

 Dispersoidchemie. 127 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 

 introduction to the physics and chemistry of colloids. 

 4th ed. 172 p. London, 1922. Other books on 

 this subject are mentioned in the List of Books, 

 at the beginning of the present volume. — Ed. 



Leipzig, 1877. 



1 Pfeffer, W., Osmotische Untersuchungen 



i The most perfect precipitation membranes yet made are chose of Morse and his coworkers, 

 who spent many years in very thorough studies on the osmotic pressures developed by con- 

 centrated solutions, the work being carried out in the Chemical Laboratory of the Johns 

 Hopkins University. Greatly improved forms of the Pfeffer cell were employed and the copper 

 ferrocyanide membranes of these writers 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 . Morse, H. N., The osmotic pressure of cane sugar solutions at high temperatures. Ibid. 



