24 ELEMENTS OF BIOLOGY 



face than elsewhere in the solution. Hence the amount of potential 

 energy at the surface of a water-alcohol solution is materially less 

 than in a surface of pure water. So when various adulterants are 

 added to water the surface energy relations are distinctly altered. 



If this is true in non-living systems, like water-alcohol solutions, 

 it is also true in protoplasm, which is at least 70 per cent water. 

 It is known that protoplasm contains and constantly manufactures 

 substances that reduce surface tension, for example, fats. Further- 

 more, protoplasm is also constantly using up and reducing the 

 amounts of such substances. Consequently, in the colloidal proto- 

 plasm the energy relations and characters of the interfaces between 

 the colloidal particles and the suspending medium are constantly 

 undergoing change; surfaces increase and decrease and the propor- 

 tions of different sorts of molecules vary as compounds are broken 

 down or as new compounds are formed. 



Electrical Charges at Surfaces. If an inorganic substance 

 such as hydrochloric acid, hydrogen chloride, is dissolved in water, 

 an appreciable number of the HCl molecules are decomposed into 

 positive hydrogen ions and negative chlorine ions. These electrical 

 charges are real, not merely theoretical. Hence in this solution there 

 are to be found water molecules, HCl molecules, positive H ions, 

 and negative CI ions. The surface film will be composed of some 

 of each of these constituents. Now it happens that the H ions and 

 the CI ions do not move about with the same speed, nor do they 

 reduce surface tension to the same degree. The H ions move much 

 the more rapidly and will be somewhat in excess at the surfaces 

 of the solution. This preponderance of H ions confers thereon a 

 positive electrical charge that may be detected and measured with 

 suitable instruments. Similarly, in all cases where a substance 

 dissociates into ions, electrical charges, either positive or negative, 

 are found at surfaces in the solution. If colloidal particles are pres- 

 ent in the solution, they too bear surface charges, since they expose 

 surfaces to the surrounding liquid that suspends them. 



