APPENDIX 593 



tected from the precipitating action of electrolytes. It has ac- 

 quired the stability of the adsorbed emiilsoid. 



Autoprotection. — Autoprotection, double and ])lural protec- 

 tions have been observed. As an example of autoprotection, 

 Alexander mentions that in pure iron "one allotrope, y - iron, 

 seems to be adsorbed by a - iron." 



Effect of Speed of Mixing*. — When oppositely charged col- 

 loids are mixed, it is quite possible for flocculation to occur as a 

 result of the neutralization of charges. If, however, an excess of a 

 positively charged emulsoid is added to a negatively charged sus- 

 pensoid, the emulsoid may be adsorbed before precipitation occurs. 

 If a small amount of the emulsoid instead of an excess is added, or 

 if the emulsoid is added very slowly, precipitation may occur be- 

 fore the protection is established. This shows that relative propor- 

 tions and speed of mixing profoundly affect colloidal reactions. 

 This is commonly given as the explanation of the zoning phe- 

 nomena seen in many immunological reactions. It can also be 

 shown experimentally that when small amounts of the protective 

 colloid added bring the other colloid to the isoelectric point, 

 precipitation commonly occurs. 



Solubility of Precipitates. — In regard to protection and protec- 

 tors, Alexander* says, "although some salts (citrates, sulpho- 

 cyanates) may act as protectors, protection is generally accom- 

 plished by adding a reversible or emulsoid colloid to an irreversible 

 one, which thereupon acquires reversible properties, that is, it be- 

 comes insensitive to electrolytes, redissolves after desiccation (at 

 any temperature that does not render the protector insoluble) and 

 passes through ultrafilters that would otherwise hold it back." 



References 



Alexander, Jerome: Colloid Chemistry, New York, 1929, D. Van Nostrand 



Co. 

 Harkins, W. D. : Atoms, Ions, Salts, and Surfaces, in Jordan and Falk, 



Newer Knowledge of Bacteriology and Immunology, Chicago, 1928, 



University of Chicago Press, p. 136. 

 Kruyt, H. E.: Colloids, Translated by H. S. Van Klooster, New York, 1930, 



John Wiley and Sons. 

 Macleod, J. J. E.: Physiology and Biochemistry in Modern Medicine, ed. 6, 



St. Louis, 1930, The C. V. Mosby Co. 

 Wells, H. G.: Chemical Patholog\^, Philadelphia, 1920, AV. B. Saunders Co. 

 Wells, H. G. : Chemical Aspects of Immunity, New York, 1925, Chemical 



Catalogue Co. 



♦Alexander, Jerome: Colloid Chemistry, D. Van No.strand Company, Inc. 

 Quoted by permission. 



