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 
virhenit 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 
