46 



GENERAL SCIENCE 



ticity of air is the property which makes it so valuable 

 as a shock absorber in pneumatic tires. If you strike 

 the handle of a bicycle pump when the outlet is closed, 

 the handle will fly back to its first position. 



Almost all solids are elastic to some extent. You 

 may test the elasticity of rubber, steel, marble, and 



wood by dropping 

 balls of these sub- 

 stances on a slab 

 of iron or stone 

 and noting the 

 height of the re- 

 / bound. 



FIG. 38. A Simple Apparatus for Testing the 

 Elasticity of a Wooden Bar. 



Experiment 4. 



Test the bending 

 elasticity of a wooden yardstick (Figure 38) by adding weights to 

 the scale pan and reading the amount of the bend from the scale. 

 Is there any relation between the amount of the bending and the 

 weights ? Give six uses of elasticity in a commercial way. 



Cohesion and Adhesion. - - The attractive force which 

 binds molecules of the same kind together is called co- 

 hesion, and the force which binds together molecules of 

 unlike kind is called adhesion. It is cohesion which 

 produces the rigidity of solids, and it is adhesion which 

 enables us to glue together two pieces of wood. This 

 distinction between cohesion and adhesion is one made 

 simply for convenience and not because the forces are 

 essentially different. We have no reason for supposing 

 that the force holding particles of wood to molecules of 

 glue is different from that holding molecules of glue 

 together. 



If a piece of plate glass is held flat on the surface of 

 water, it will require some force to remove it (Figure 39) . 



