298 PHYSICAL PROPERTIES 



the colloid network in which they are entangled necessarily 

 compel the compensating migration of water. No electro- 

 static tension between the jelly and the external solution need 

 be assumed and since no acid anions are yielded by the protein 

 salts, simultaneous equality of concentration of the uncombined 

 acid and the acid anions within and without the jelly will be 

 assured by their normal and equal diffusion into the jelly. The 

 increased swelling capacity of gelatin in solutions of acids or 

 alkalies is merely the expression of the fact that the ionization 

 of the protein salt leads to an increase in the number of colloid 

 particles per unit volume of the jelly and possibly also in part 

 of the fact that protein ions have a greater affinity for water 

 than undissociated protein molecules. 



This conception of the process of swelling would still yield 

 no equilibrium or swelling-maximum were there no compensating 

 force acting in an opposite sense to the osmotic pressure of the 

 gelatin. Since gelatin plates when immersed in water do not 

 swell indefinitely until swelling merges insensibly into solution 

 but, on the contrary, display a more or less well-marked swelling- 

 maximum, the osmotic pressure exerted by the colloid particles 

 within the jelly must, upon attainment of this maximum, be 

 balanced by an equal opposing force which Procter interprets 

 as the tension of the elastic colloid network (77). Applying 

 Hooke's law,' Procter finds that this tension (= E) is defined 

 by the equation : 



TjJ C 1 I V - 



\ sp. gr. of gelatin/ ' 



where E is the stress inducing swelling, V is the volume attained 

 by one gram of gelatin, and C is the modulus of elasticity. He 

 finds that C = 0.00125 at 7 degrees; 0.00055 at 15 degrees; and 

 0.00021 at 18 degrees. The value of E at first increases with 

 acidity to a maximum and then decreases, slowly approaching 

 zero. 



It has been shown by Ostwald (64) and Chiari (16) that gelatin 

 in very faintly acid media displays a well-marked minimum of 

 swelling-capacity. This minimum occurs at a hydrogen ion con- 

 centration almost exactly coinciding with the hydrogen ion 

 concentration of gelatin solutions in which the gelatin is "iso- 

 eleetric," i.e., does not wander in an electrical field. In other 



