THE PROPERTIES OF MATTER. 15 



of changing its condition of rest or motion, or the 

 property by virtue of which it has a tendency when at 

 rest to remain at rest, or when in motion to continue in 

 motion. 



(a.) If a ball be thrown, it requires external force to put it in mo- 

 tion; the ball cannot put itself in motion. When the ball is passing 

 through the air it has no power to stop, and it will not stop until 

 some external force compels it to do so. This external force may 

 be the bat, the catcher, the resistance of the air, or the force of 

 gravity. It must be something outside the ball or the ball will move 

 on forever. Illustrations of the inertia of matter are so numerous 

 that there should be no difficulty in getting a clear idea of this 

 property. The " running jump " and "dodging" of the play- 

 ground, the frequent falls which result from jumping from cars in 

 motion, the backward motion of the passengers when a car is sud- 

 denly started and their forward motion when the car is suddenly 

 stopped, the difficulty in starting a wagon and the comparative ease 

 of keeping it in motion, the " balloon" and " banner" feats of the 

 circus-rider, etc., etc., may be used to illustrate this property of 

 matter. 



39. Experiment.^ Upon the 



tip of the fore-finger of the left 

 hand, place a common calling-card. 

 Upon this card, and directly over 

 the finger, place a cent. With the 

 nail of the middle finger of the 

 right hand let a sudden blow or " snap " be given to the 

 card. A few trials will enable you to perform the experi- 

 ment so as to drive the card away, and leave the coin 

 resting upon the finger. Repeat the experiment with the 

 variation of a bullet for the cent, and the open top of a 

 bottle for the finger-tip. 



40. What is Mobility ? Mobility is that prop- 

 erty of matter by virtue of which the position of 

 bodies may be changed. 



