4 THE DOMAIN OF PHYSICS. 



Molar attraction is called gravitation. 



Molecular attraction is called cohesion or adhe- 

 sion. 



Atomic attraction is called chemical affinity (chera- 

 ism). 



8. Forms of Motion. Each of these three divi- 

 sions of matter has its own form of motion : 



Molar motion, or visible mechanical motion, is called 

 by different names according to the nature of the 

 substance in motion ; e. g., the flow of a river or the 

 vibrations of a pendulum. 



Molecular motion, called heat, light, electricity, or 

 magnetism. 



Atomic motion. (Purely theoretical as far as known.) 



9. Physical Science. Physical science com- 

 prises Physics and Chemistry. 



The first of these deals with masses and molecules; the 

 second with atoms and combinations of atoms. 



1C. What is a Physical Change ? A physi- 

 cal change is one that does not change the identity 

 of the molecule. 



(a.) Inasmuch as the nature of a substance depends upon the 

 nature of its molecules, it follows that a physical change is one that 

 does not affect the identity of a substance. A piece of marble may 

 be ground to powder, but each grain is marble still. Ice maj 

 change to water and water to steam, yet the identity of the sub- 

 stance is unchanged. A piece of glass may be electrified and a 

 piece of iron magnetized, but they still remain glass and iron. These 

 changes all leave the composition and nature of the molecule un- 

 changed ; they are 'physical changes. 



11. What is a Chemical Change ? A chemi* 



