I04 



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^ 

 employed porous clay cylinders such as are 

 used in electric batteries. When such a 

 Dorous cell is filled with a copper sulphate 

 (CUSO4) solution and placed in a solution 

 of potassium ferrocyanide (K^Fe-CCNje), a 

 membrane of copper ferrocyanide Cu2 Fe- 

 (CN)6) is precipitated in the porous wall. 

 Similar precipitation membranes may be ob- 

 tained with other substances, such as iron 

 sihcate. 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.' 



' 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 con- 

 centrated 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 im- 

 permeable 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. 

 48:29-94. 191 2. Idem, The osmotic pressure of aqueous solutions. Carnegie Inst. Wash. 

 Pub. 198. 222 p. 1914. During the same period other very important experimental studies 

 on the osmotic pressure developed by concentrated solutions have been prosecuted by Berkeley 

 and Hartley, in England. See: Berkeley, Earl of, and Hartley, E. G. J., On the osmotic 

 pressure of some concentrated solutions. Phil, trans. Roy Soc. London A206: 481-507. 

 1906. For a general discsusion, see Findlay, 1913, also Washburn, 1915. (See note f, p. 

 xoi.)— Ed. 



Fig. 67. — Pfeffer osmometer (z), 

 with closed mercury manometer. 

 {After Pfeffer.) 



