256 THEORY OF COLLOIDAL BEHAVIOR 



salts on colloidal solutions or suspensions is the result of the 

 Donnan equilibrium, the stability of such solutions must be due 

 to the fact that when isolated protein ions are beginning to 

 coalesce a Donnan equilibrium is set up between the solution 

 inside each nascent micella and the outside solution, which 

 results in a swelling of the nascent micella whereby the ions in the 

 process of coalescence are forced apart again. This prevents the 

 formation of new micellae from protein ions as well as the coales- 

 cence into larger complexes of the micellae already existing. More- 

 over, there must also originate a P.D. between the micella and 

 the solution, and the mutual repulsion of the micellae due to their 

 electrification will also prevent the coalescence of individual 

 micellae into a precipitate. If a salt is added, the forces guaran- 

 teeing the stability of the colloidal solution, e.g., the osmotic 

 pressure, swelling, and P.D. of the micellae are diminished. 

 When these forces fall below a certain minimal value the protein 

 particles will coalesce. 



We have seen that the depressing effect of a salt on swelling, 

 osmotic pressure, and P.D. of protein particles is due to that ion 

 of the crystalloidal salt which has the opposite sign of charge to 

 that of the protein ion; and that the depressing effect of this 

 crystalloidal ion increases with its valency. Thus, Fig. 76 

 indicates the depressing effect of different concentrations of NaCl 

 and Na 2 SC>4 on osmotic pressure and P.D. of a 1 per cent gelatin 

 chloride solution of pH of originally 3.5. The abscissae are the 

 concentration of the salt added, the ordinates the osmotic pressure 

 and P.D. The figure shows that the depressing effect of the same 

 molecular concentration of Na 2 SO 4 is much more than twice as 

 great as the depressing effect of NaCl. If we assume that the 

 protein ions and protein micellae can coalesce when the osmotic 

 pressure is 100 mm. and the P.D. about 4 millivolts, this low 

 osmotic pressure and low P.D. of the 1 per cent solution of gelatin 

 chloride of pH originally 3.5 will be produced when the NaCl 

 solution is about M/64 and the Na 2 SO 4 about M/512. The 

 precipitating effect of Na 2 SO 4 on gelatin chloride would then be 

 about eight times as great as the precipitating effect of NaCl. 

 The depressing effect of CaCl 2 on the osmotic pressure, swelling, 

 and P.D. is about the same as that of a NaCl solution of the same 

 concentration of chlorine ions, showing that the depressing effect 



