APPENDIX 589 



lost their charge, they will not travel to either pole. This state of 

 affairs exists at the "isoelectric" point where neutralization of 

 charge has occurred. By cataphoretic experiments the nature of 

 the charge of various colloids has been determined. 



Adsorption. — Adsorption and Surface Affinities. — It is gen- 

 erally known that finely powdered charcoal will remove colored sub- 

 stances that are present in a solution. The intensity of color of 

 certain dye solutions is markedly reduced by charcoal. In each 

 case, the material removed has attached itself to the surface of the 

 finely dispersed particles of charcoal. This plienomenon is called 

 adsorption and is a characteristic of all matter in the colloidal 

 state. 



Positive Adsorption. — The term positive adsorption means that 

 there has occurred an increase in concentration of some substance 

 in the boundary layer of the dispersed substance and dispersion 

 medium. 



Surface Concentration of Dissolved Substance. — Willard Gibbs 

 called attention to a principle that is perha])s fundamental to 

 adsorption, and that is, that when a substance is dissolved in a 

 liquid and lowers its surface tension, it appears in greater con- 

 centration at the surface. It is now generally thought that the 

 way the surface molecules are oriented is responsible for many 

 of the properties of adsor])tion. 



Surface Wetting. — It is readily appreciated that when oil or 

 fat is dispersed in water the water is unable to wet the greasy 

 surface of the dispersed fat droplets. On the other hand, if oil 

 droplets can be filmed with proteins, the surfaces may readily be 

 made wet with water. 



Devaux Experiment. — Alexander cites an experiment of De- 

 vaux which seems to explain the phenomenon of surface wetting. 

 Devaux floated a drop of molten fatty acid on the surface of some 

 hot water and allowed it to cool. The upper surface was in contact 

 with the air, while the lower was adjacent to the Avater. He found 

 that after he dried the lower surface he coiild wet it with water 

 but that he could not wet the upper surface with water. Alexander 

 says that apparently the "hydrophile" or "friendly-to-water" 

 ends of the fatty acid molecules were turned toward the surface 

 that cooled in contact with the water, while the "hydrophobe" or 



