110 PROTOPLASM 



Numerous factors influence the stability of colloidal particles 

 to a greater or lesser degree. Charge is the primary one. In 

 the case of most solid (metal) suspensions, it predominates, 

 but in the case of organic and certain inorganic suspensions, 

 hydration is also an important factor in stability, though 

 charge is ejffective here too. The maximum charge necessary 

 is variable. Northrop and de Kruif found 15 mv. to be the 

 critical potential (see page 376) of certain bacteria; but under 

 other conditions {e.g., when Na2HP04 is present), the poten- 

 tial is higher. Other bacteria may remain in suspension 

 ^ @ when their potential is reduced to zero. 



® @M.^ ®^^ Here, hydration is probably respon- 

 ® %^rvtr^^^ ® sible, as in the case of proteins. Still 

 ® ^^f'VohV ''fP ® other strains of bacteria, studied by 

 ®^k'-'' ' ■ ''-7^ &^ ^* Mudd, also show zero potential, 

 ® ^#^^±i±^i^^^^ @ though stable over a wide range in 

 ® ^^"^„ acidity; this can mean only an un- 



® usual type of surface, possibly a carbo- 



hydrate. Stability would here be due to 

 pfjj CI hydration. 



YiG. 80.— The ionic en- Electrokinetic potential is the chief 

 vironment of a colloidal factor determining stability, but impor- 

 partic e. } er am.) ^^^^ ^^^ hydration (solvation), Brownian 



movement, absence of cohesion, convection, and size; of 

 minor significance are shape, and temperature. 



Precipitation. — The formation of deltas at the mouths of rivers 

 is an example of the precipitation of colloidal matter. When 

 the muddy waters of the river reach the sea, the fine clay particles 

 in suspension are precipitated by the salts of the sea, and deltas 

 result. If salt or other electrolytes are added to a colloidal 

 solution in a test tube, precipitation ordinarily takes place. 

 If a colloidal suspension of negative charge, such as silver, is 

 added to one of positive charge, such as iron hydroxide, both are 

 precipitated. The opposite charges have neutralized each 

 other. Two positive or two negative colloids do not (usually) 

 precipitate one another. Precipitation by electrolytes is due to 

 neutralization of the charge on negative colloidal particles by 

 positive cations (Na+, Ca++, A1+++), and of the charge on posi- 

 tive colloidal particles by negative anions (Cl~, SO4 , PO4 ). 

 Precipitation and the closely related processes of coagulation. 



