ENERGY AND FORCE 



47 



FIG. 39. The glass plate ad- 

 heres to the water. 



If we examine the glass after it has been removed, we shall 

 find it wet, showing that when we lifted the glass plate, 

 we separated water molecules from water molecules 

 and not water from glass. This 

 force may be measured easily by 

 arranging a balance, as shown in 

 Figure 40, and determining the 

 weight that is required to lift the 

 plate of glass and break the co- 

 hesive force. The cohesive force 

 will be this weight less the actual 

 weight of the plate of glass. If 

 the size of the plate is known, 

 the force per square inch may 

 be easily determined. 



Shape of Free Liquid. Small 

 drops of a liquid are spherical in shape, for the surface 

 of a sphere is the minimum surface for a given mass, and 

 the cohesive forces, acting between the molecules of a 



given mass of liquid, tend to 

 reduce it to the volume hav- 

 ing the smallest possible sur- 

 face. In larger quantities of 

 liquids, the force of gravita- 

 tion is large enough to be 

 more of a factor in determin- 

 ing the shape than the co- 

 hesive forces, hence the drops 

 flatten out. 



On the surface of a liquid 

 the attractive forces between 



FIG. 40. Measuring Cohesive Force. 



the molecules are sufficiently strong to form a film. A 

 needle which is much heavier than water may be floated 



