154 GENERAL SCIENCE 



Inertia as another of the properties of matter refers to the 

 fact that bodies at rest cannot set themselves in motion, and 

 that when in motion they cannot stop themselves. A body 

 in motion presupposes some force having acted upon the body 

 to cause the motion. On the other hand it is a matter of 

 common experience to note that moving bodies come to rest. 

 Whatever their tendency to keep moving, i.e., their inertia 

 as moving bodies, they are brought to rest after a time by 

 reason of some form of resistance experienced by them when 

 moving. 



Only as sufficient force is applied to make good the loss of 

 energy as a moving body does work in overcoming resist- 

 ance will the motion of that body be maintained. This 

 resistance may be that of friction developed as one surface 

 slides upon another; or the resistance of medium as when a 

 body moves through the air, or through a liquid or a solid; 

 or it may be due to the opposing action of some other force. 



That the earth in its revolution about the sun not only 

 continues its journey by reason of the action of some initial 

 force long since spent, but at a rate which makes the years 

 as periods of its revolution of a uniform length, argues that 

 with its enveloping atmosphere the earth sweeps through 

 spaces destitute of any matter, even of the most rarefied gase- 

 ous nature, and so meets no resistance to its motion. 



When a body is acted upon in any such way as to cause a 

 change in its form without a change in its position, it is said 

 to be under a strain. The body is more or less deformed. 

 The force causing a strain, whatever the force may be, is 

 called a stress. The two terms are correlative, i.e., the use 

 of one of them implies the other. 



The properties denned above are characteristics of all 

 matter whether in solid, liquid, or gaseous state. They are 

 the so-called general properties. There are also specific 

 properties of matter characterizing certain bodies only. 



