THE RECIPROCAL RELATION BETWEEN THE OSMOTIC 



PRESSURE AND THE VISCOSITY OF GELATIN 



SOLUTIONS. 



By JACQUES LOEB, 

 {From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, July 20, 1921.) 



1. If we put a suspension of 1 gm, of finely powdered gelatin in 100 

 cc. of HCl, so that the gelatin has a pH of 3.4, into a collodion bag 

 and dip the latter into a pure solution of HCl (without gelatin) of 

 pH 3.0, the liquid will practically not rise in the manometer connected 

 with the liquid in the collodion bag (except to that slight extent which 

 may be caused by some of the solid gelatin going into solution) ; in 

 other words, we get no osmotic pressure. The reason is as follows: 

 in this experiment the Donnan equilibrium is established between 

 each individual granule of solid gelatin and the solution of HCl inside 

 the bag and since the latter diffuses freely through the membrane 

 there is in this case no difference of pH inside and outside the collo- 

 dion membrane, and hence no difference of osmotic pressure on the 

 opposite sides of the membrane (unless some of the solid gelatin 

 goes into solution) . 



If we make this experiment, however, with a 1 per cent solution 

 of gelatin chloride of a pH of 3.4 inside the collodion bag, while on 

 the other side of the membrane we have a HCl solution of pH 3.0 

 without gelatin, we notice a rapid rise of the column of water in the 

 manometer tube inserted into the collodion bag and the osmotic 

 pressure of the 1 per cent solution of gelatin will be about 450 mm. 

 H2O or a little more when equilibrium is established.^ This proves 

 that the osmotic pressure of a gelatin solution cannot be due to 

 (submicroscopic) solid particles of a jelly, but must be due to particles 

 causing a Donnan equilibrium across the collodion membrane. Such 

 particles can only be the individual gelatin ions, since even the smallest 



' Loeb, J., J. Gen. Physiol, 1920-21, iii, 691. 



97 



