272 CHEMISTRY OF THE PROTEIDS CHAP. 



" (2) Indirect action of a kat-ion radical on the colloid : 



+ + 



Colloid 4- water + HC1 = (colloid) (water) H + 01. 



[In this case the colloid acts as a dielectricon or a body which has 

 no electrical charge of its own, but in which an electrical charge may be 

 induced owing to the colloid being a hydrate. The author holds further 

 that the development of a charge in a ' dielectricon ' is accompanied by 

 some kind of definite chemical change in the dielectricon.] 



" (3) Direct action of an an-ion radical on the colloid : 



+ - + 

 Colloid + HC1 + water = (colloid) Cl H + water." 



"In this last case the chlorine-ion, which has greater negative 

 electro-affinity than the hydrogen-ion has a positive-affinity, may be 

 assumed to satisfy its full negative-affinity by evoking a positive 

 response of the colloid, in which case the positive H and the positive 

 colloid will together completely balance the negative Cl'." [See also 

 the author's definition of a neutral salt, p. 265 of this book.] 



The changes which colloids defined in the manner just given 

 may undergo, were then classified in this way : l 



" (A) Changes in temperature leading to ' setting ' : 



1. By lowering the temperature. 



2. By raising the temperature. 



" (B) Mechanical shaking producing ' conglutination ' : 

 " (C) Physical factors inducing ' coagulation ' : 



1. By alterations in the electrical tension between the colloid 



and its solvent. 



2. By dehydration (or ' salting out '). 



" (D) Chemico-physical factors causing 'precipitation/ owing to 



1. A withdrawal of the H or OH' radical of the colloid. 



2. A removal of salts. 



3. The formation of insoluble or irreversible salts. 



4. Heat action. 



" (E) Chemical action accompanied by physical change, owing to. the 

 formation of additive compounds between colloids and non-electrolytes 

 by the process of oxidation or reduction." 



This classification has again been adopted in the following pages, 

 with the addition of a chapter on so-called spontaneous coagulation on 

 p. 323. (See also p. 266.) 



1 Mann, PhysioL HistoL p. 48. 



