PHYSICAL FORCES INVOLVED IN BIOLOGICAL ACTIVITIES 169 



over which an influence of attraction is exerted would be in the form 

 of a sphere and would be recognized as the sphere of molecular action. 

 The layer of a liquid representing its surface plane with a depth 

 equal to the radius of the sphere of molecular action would be the surface 

 film. If a particle lies within or inside of this surface film it follows 

 that with this particle as a center, the radius of its sphere of activity 

 will extend beyond and above the surface film, but if this particle lies 

 without and below this surface film the molecular forces on all sides will 

 be equal and an equilibrium established. 



FIG. 103. Illustrating surface forces. 



This is illustrated in Fig. 103. AB is the plane surface of a liquid. 

 N is a particle with its circumference indicated in which all forces are 

 equalized. N' is a particle in which the forces downward are greater 

 than the forces upward. The forces lying above the plane surface of 

 the liquid AB appear to be less than the forces operating immediately 

 below the plane surface AB in the liquid, yielding a considerable 

 increase of pressure in the liquid. This increased pressure is known 

 as the molecular pressure of the liquid. 



The surface film described above possesses a pull or is under tension 

 or is the surface tension of the liquid. If an iron ring has stretched across 

 its interior surface a soap film and a silk-thread loop is carefully rested 

 upon it and run to the iron ring, the film inside the silk loop may be 

 broken readily by any penetrating substance when the sides of the loop 

 will spread out in the fullest degree drawn by the soap film without. 

 Much like this is the floating of a rubber band on water. If a rod 

 dipped into alcohol is touched to the surface of the water within the 

 band the water film without pulls the band into its full circular form 

 (Fig. 1046) through the reduction of the surface tension of the water 



