VISCOSITY 



213 



of a continuous jelly is preceded by the formation of submicro- 

 scopic pieces of jelly, which increase in number and size, forming 

 finally a continuous jelly. Hence, the longer a solution of 

 isoelectric gelatin stands at 20C. the greater the number of 

 submicroscopic solid pieces of jelly formed in the solution. The 

 submicroscopic pieces of jelly, surrounded by a true solution of 



pHl.6 18 2.0 22 24 2.6 25 3.0 3.2 3.4 3.6 3.8 40 42 44 



FIG. 56. Increase in viscosity when acid is added to solutions of isoelectric gela- 

 tin after they had been standing for 3 and 17 hours respectively. 



isolated molecules of gelatin in water, are compelled to regulate 

 the amount of water they occlude by the Donnan equilibrium. 

 Hence, when we add some HC1 to a 0.5 per cent solution of iso- 

 electric gelatin after the solution has been standing for some 

 hours at 20 we should expect to find a higher viscosity than when 

 we add the same amount of acid to the gelatin solution immedi- 

 ately after it has been rapidly heated to 45 and rapidly cooled 

 to 20C. 



This experiment turns out as expected, as is shown in Fig. 56. 



