6 PHYSICS. 
8. Elasticity.—This property is intimately connected with porosity and 
density. We have seen that bodies are both compressible and dilatable ; 
now, some bodies when compressed have a natural tendency, when the 
pressure is removed, to expand again to their original volume ; such 
bodies are said to be elastic, while those that retain the form given them 
by the compressing force are called non-elastic. Elasticity is not con- 
fined, however, to bodies that may be compressed like a piece of 
sponge, but those bodies are also said to be elastic, which return 
to the state or position in which they were before the force changing 
them was applied. A steel spring, when it is bent and then let go, 
immediately springs back to its former position; so. with a piece of 
india-rubber when stretched. The most elastic of all bodies are gases. 
The air-gun affords a very good illustration of the elasticity of common 
air. At the breech of a gun, with an. ordinary barrel, is a small hollow 
sphere, into which air is forced by an instrument for the purpose until it 
is very much condensed. The opening by which the air is forced in is 
closed by a valve which opens inward, and when a shot is to be fired, 
the cock strikes the valve in, allowing a quantity of the condensed air 
to escape into a chamber behind the ball; and so great is the elasticity of 
the air, that the ball is projected from the gun itl a force aor equal 
to that of a charge of gunpowder. 
9. Inertia. _Inertia i is the property which bodies have of always remain- 
ing in the same state, till that state be changed by external causes. Bodies 
at rest will remain eae at rest, and bodies 3 in motion will remain always 
in motion, as far as they themselves are concerned ; that is, till they be 
set in motion or stopped by external causes. No proof is required 
of the fact that when a body is at rest, it will not begin to move 
of itself; but it is not so clear that when a body is in motion, it 
will not stop of itself: indeed, many phenomena seem to prove the 
contrary. However powerfully a stone may be thrown or rolled along 
the ground, it will at last come to rest. It would thus seem as if 
it ceased to move of itself; but the truth is, that two very powerful 
agents are at work to stop it. These are the resistance of the air and 
friction. The resistance of the air is far greater than is generally 
supposed, On putting one’s head out of the window of a railway train, 
the effect experienced is exactly the same as if a gale of wind were 
blowing. It has been calculated that a 24-pound cannon-ball, when 
fired with a velocity of 2000 feet per.second, experiences a resistance 
of 800 pounds. ‘The retarding force of friction is greater even than this. 
We best see its effect by considering what takes place when it is removed. 
It is well known that a ball will roll much farther, with the same force, on 
a. smooth floor than on a rough piece of ground, and much farther on a 
sheet of ice than on either. The smoother the surface, then, on which a 
