212 ANIMAL PROTEINS 



Now with all electrolytes, even with water, we have both 

 positively and negatively charged ions, and y is consequently 

 determined by the difference in the amounts adsorbed. 

 Hence in the case of an electrolyte with an equal number of 



oppositely charged ions^y = ma^c n \ ma 2 c n z, where a\, a 2> and 

 HI, n 2 , are the appropriate constants for the particular ions 

 concerned. Hence at constant temperature, pressure, etc., 

 we may write 



s 

 F = 



The force tending to make a piece of gelatine swell is pro- 

 portional to its mass, which is perhaps fairly obvious. The 

 swelling force is also an inverse function of the volume of the 

 gel, and as swelling proceeds therefore the force tending to 

 swell further decreases. The force tending to swell is pro- 

 portional to the specific surface of the disperse phase, other 

 factors being constant. To illustrate this one has only to 

 imagine that one particle of the disperse phase be split into 

 two particles each carrying half the original charge. It is 

 clear that a new repulsive force becomes operative, which 

 did not before influence the swelling, and that the distance 

 between the particles is halved. In the swelling of gelatine, 

 however, we may consider the dispersity constant for 

 constant temperature, and if we consider unit mass we see 

 that the force causing swelling depends upon the operation 

 of the adsorption law and upon the degree to which the gel 

 is already swollen. 



In the swelling of (say) one gram of gelatine to its 

 maximum, both the contractile force of surface tension and 

 the expanding force of electrical repulsion are in operation. 

 At the commencement the latter is much the greater force 

 hence the rapid imbibition. Both these forces decrease in 

 magnitude as the swelling proceeds, but the force tending to 

 swell decreases at a more rapid rate, and the time comes 

 when it has decreased to the precise value of the force tending 

 to resist swelling. At this point equilibrium is established 

 and the maximum swelling attained. Obviously this 



