PROTOPLASMIC STRUCTURE 85 



negatively charged surfaces; e.g., silicates, carbon, and 

 adsorbents of chemically acid character, but not by 

 hydrates like alumina or other positive adsorbents.^ 

 Suspensoids of heat-denatured albumin show the same 

 behavior; when the particles are made positive by the 

 addition of a little acid they become adsorbents for 

 acid dyes, which also cause precipitation; while in the 

 negative condition, i.e., on the alkaline side of the iso- 

 electric point, they are precipitated by (and adsorb) 

 basic but not acid dyes. The mutual precipitation of 

 oppositely charged colloidal solutions when mixed in 

 suitable proportions is an example of the same phe- 

 nomenon. 



The importance of the electrical factor in the general 

 behavior of colloids is indicated by the remarkable 

 changes in adsorptive and chemical properties which 

 a given protein exhibits when the H-ion concentration 

 of the solution passes from one side of the isoelectric point 

 to the other. On the acid side precipitation is induced 

 by the anions of an added electrolyte, on the alkaline 

 side by the cations, as Hardy first showed in the case of 

 heat-modified egg-albumin.^ Recently the relation 

 between the charged condition of the protein aggregates 

 and their chemical and other behavior has been investi- 

 gated in much detail by Loeb,^ who has determined many 

 striking correlations between the physical and the 

 chemical properties of proteins at varying H-ion con- 



^ Cf . Michaelis, Physikalische Chemie iind Medizln, edited by Koranyi 

 and Richter, Leipzig, II (1908), 341. 



^^ W. B. Hardy, Proceedings of the Royal Society, LXVI (1899), no. 



3 For a summary of these important investigations cf . Loeb's recent 

 book Proteins and the Theory of Colloidal Behavior, New York, 1922. 



