264 THEORY OF COLLOIDAL BEHAVIOR 



We conclude from these experiments that gelatin forms a 

 colloidal suspension in a mixture with much alcohol and little 

 water and that the stability of the suspension depends in this 

 case upon the forces set up by the Donnan equilibrium between 

 the micellae and the surrounding liquid, these forces being osmotic 

 pressure and P.D. 



In aqueous solutions or in solutions with little alcohol and much 

 water the stability of the gelatin solution depends on forces which 

 have no connection with the Donnan equilibrium and which may 

 be the forces of secondary valency between gelatin ions or mole- 

 cules and the molecules of water which are supposed to be respon- 

 sible for the stability of crystalloidal solutions in general. 



These forces of secondary valency do not cease to exist (though 

 they are weak) in solutions with much alcohol and little water, 

 and these forces may contribute also to the stability of the solu- 

 tion. This seems to be indicated by some of the data in Table 

 XL VIII. This table shows that M/10 NaCl precipitates gelatin 

 from the solution in much alcohol and little water. If the forces 

 due to the Donnan equilibrium alone determine the stability of 

 the suspension, a M/20 CaCl 2 solution and a M/30 LaCl 3 solution 

 should have the same effect, since only the anion acts in the case 

 of the Donnan effect when gelatin is positively charged. Table 

 XL VIII shows that the CaCl2 and LaCls solutions required for 

 precipitation are higher than M/20 or M/30, namely M/12 for 

 CaCl 2 and M/ll for LaCla. This means that Ca and still more 

 La have an inhibiting effect on the precipitation. We have seen 

 that Ca and La increase the solubility of isoelectric gelatin in 

 water, i.e., they increase the forces of attraction between water 

 and gelatin (see Table XL VIII). It is possible that the inhibition 

 of the precipitating effect of Cl by La and Ca is due to the 

 augmenting effect of these cations on the solubility of gelatin in 

 water. This inhibiting effect on precipitation is often spoken of 

 as the peptization effect. While the precipitating effect is due to 

 the action of salts on the Donnan equilibrium, the peptization 

 effect seems to be due to the influence of salts on the secondary 

 valency forces between molecules of gelatin and solvent, in these 

 experiments at least. 



A few remarks may be added concerning the precipitation of 

 crystalline egg albumin from aqueous solutions by salts at room 



