COLLOIDAL SUBSTANCES 277 



2. This depressing effect of the salt is always due to that ion 

 which has the opposite sign of charge to that of the colloidal 

 particle. 



3. The depressing effect increases with the valency of the 

 effective ion. 



4. When the colloidal substance used is an amphoteric electro- 

 lyte the addition of little acid or alkali to the isoelectric substance 

 increases the osmotic pressure, P.D., viscosity, stability of the 

 solution, and swelling of gel until a point is reached where the 

 further addition of acid or alkali will have the opposite effect. 



5. The depressing effect of an excess of acid or alkali is also due 

 to the ion which has the opposite sign of charge to that of the 

 colloidal particle, and the efficiency of that ion also increases 

 with its valency. 



It is obvious that the stability of colloidal solutions is only one 

 of a number of properties which all possess the same characteristic 

 features. It has been shown in the preceding chapters that 

 all these characteristics find their explanation in the Donnan 

 equilibrium. 



If, on the basis of this knowledge, we continue the mode of 

 reasoning expressed in the quotation from Burton, we come to the 

 further conclusion that it is no longer correct to discriminate 

 between the colloidal and the crystalloidal state of matter, for 

 the same substance may behave in the same state either like a 

 colloid or a crystalloid. We have seen that a 1 per cent solution 

 of crystalline egg albumin in water at room temperature and at a 

 pH much above 1.0 will behave like a colloid or as if it were in the 

 colloidal state in regard to osmotic pressure or in regard to P.D. 

 when separated from water by a collodion membrane; but the 

 same solution of crystalline egg albumin will behave like a 

 crystalloid or as if it were in the crystalloidal state in regard to 

 the stability of the solution and practically also in regard to 

 viscosity (as long as the temperature and the concentration of 

 the solution are not too high and the pH not too low) . The reason 

 for this is plain in the light of the preceding chapters. Proteins 

 (and, perhaps, substances in general) will show colloidal behavior 

 when the following two conditions are fulfilled: first, the sub- 

 stance must be capable of dissociating electrolytically, and second, 

 one of the two oppositely charged ions must be prevented from 



