MATTER AND FORCE 155 



These are dependent upon molecular forces acting within 

 the body, and are attributes more especially of solids. Of 

 these only the following are defined: 



Tenacity refers to that property manifested by a body in the resistance 

 it offers to being pulled, bent, or twisted apart. A dry dead twig may 

 easily be broken, but the toughness of the fibres of flax and of other tex- 

 tiles makes possible their incalculable value to mankind in fabrics and 

 cordage. 



Elasticity manifests itself in a tendency in bodies when released from a 

 strain to return to an original form or volume. Bodies may be flexible or 

 compressible, they may be pulled or twisted into new forms, but it is in 

 the tendency to recover from any such strain that their elasticity is mani- 

 fest. Bodies such as wet clay which may be shaped into desired forms 

 that are retained by reason of lack of elasticity in them are said to be 

 plastic, 



When a body falls we " explain" it by saying that it is 

 caused by the attractive force of the earth known as gravity. 

 This pull of the earth is exerted not only upon bodies at or 

 near its surface but it reaches outward through all space, 

 attracting all other bodies in the universe. Sir Isaac Newton 

 (1642-1727) formulated the belief now accepted as statement 

 of a great general truth that every particle of matter in the 

 universe attracts every other particle in the same manner as 

 the falling body is attracted by the earth. This universal 

 pull of bodies upon one another is called the force of gravita- 

 tion. Little more is known about it than in Newton's day, 

 and what causes it no one knows. When the attraction is 

 between the earth and bodies at or near its surface, the 

 term gravity is more commonly employed. The molecular 

 forces of cohesion and adhesion, acting between the particles 

 of bodies at insensibly small distances, are likewise attrac- 

 tive forces binding matter together into masses. Gravitation 

 acts, however, through measurable (sensible) distances and 

 throughout the extent of the universe. 



It was part of Newton's statement ("law") concerning 

 gravitation that its value was affected both by the masses of 



