88 DONNAN EQUILIBRIUM AND VISCOSITY 



constituents, namely, first, solid submicroscopic (and microscopic) 

 particles in suspension which in due time would have settled, and 

 second, isolated casein ions and molecules. The solid particles in 

 the supernatant liquid (unless they are below the limit required 

 to occlude water) undergo the same swelling under the influence of 

 the Donnan equilibrium as the particles of the sediment. In addition, 

 however, we have individual casein ions in solution (the molecules 

 being probably insoluble since isoelectric casein is practically insol- 

 uble) but these ions cannot undergo any swelling and hence do not add 

 materially to the volume and the viscosity. As a consequence the 

 more solid particles of casein chloride are dissolved into isolated casein 



1.0 



pHl.4 1j6 1.8 2.0 ZZ 24 2.6 28 3.0 3.2 3.4 3.6 a8 40 42 



Fig. 9. Viscosity of 1 per cent casein chloride solutions after 1 and 22 hours at 

 20°C. 



ions or particles too small to occlude water the more the relative vol- 

 ume occupied by the casein in the solution should be diminished and 

 this should be accompanied by a diminution in viscosity. If our 

 theory of the origin of the viscosity of the gelatin solutions is correct, 

 it should be possible to prove that where the solubility of the casein 

 chloride solution is a maximum the viscosity curve should show a 

 drop. 



The correctness of this inference is supported by the viscosity 

 curves in Fig. 9 which represent the viscosity after 1 hour and after 

 22 hours. The experiments are the same as those referred to in 

 Figs. 7 and 8. The viscosity of the total suspension and solution 



