16 



PHYSICAL PROPERTIES. 



been closed up, and others have broken out. Rivers 

 have deserted their channels, and have been reborn 

 elsewhere. Islands have become connected with 

 islands, as Leucadia and Sicily. Land has been 

 submerged by earthquakes, and plains have been 

 upreared into hills. There was a time when Etna 

 was not a burning mountain, and the time will come 

 when it will cease to burn." 



The last property of matter which we shall no- 

 tice here is inertia. By this we mean that matter 

 is inactive ; it has neither the power to move nor 

 to stop its motion ; in short, is incapable of sponta- 

 neous change. This is equivalent simply to saying, 

 that mere matter is destitute of life ; it has no prin- 

 ciple of action in itself; and, accordingly, common 

 sense teaches us, if we see an inorganic body in 

 motion, to attribute it to some cause extraneous to 

 itself. If such a body be once put in motion, it 

 must always continue in motion, unless resisted by 

 some external cause ; it has no more power to stop 

 than to commence running. Indeed, the same causes 

 which destroy motion in one direction, are capable 

 of producing as much motion in the opposite direc- 

 tion. Thus, if we stop a wheel spinning on its 

 axis by seizing one of its spokes, we find that it 

 requires the same effort as it would have done had 

 the wheel been at rest, to put it in motion in the 

 opposite direction with the same velocity. So, if a 

 carriage drawn by horses be in motion, the same 

 exertion of power in the horses is necessary to stop 

 it, as would be necessary to back it if it were at 

 rest. It follows, therefore, from this reasoning, 

 that a body which can destroy or diminish its own 

 motion, can also put itself in motion from a state of 

 rest. But as this is manifestly impossible, so also 

 is the other. Thus the heavenly bodies continue 

 to roll on in their appointed paths of infinite space 

 with unerring regularity, preserving without dim- 

 inution all that motion which they first received 



