[Illustration: Fig. 29.—Shock communicated by elasticity.] INERTIA is the passiveness of Matter. This perfect indifference to either rest or motion makes the great distinction between living and lifeless matter. Inertia, or _Vis Inertia_, is this passiveness. Now, to overcome this indifference we must use force, and when we have applied force to matter we set it in motion; that is, we move it. When we move it we find a certain resistance which is always proportionate to the force applied. In mechanics this is termed _Action_, and _Reaction_, which are always equal forces acting in opposite directions. This is Newton’s law, and may be explained by a “weight” on a table, which presses against the table with the same force with which the table presses against the “weight”; or when you strike a ball, it strikes the hand with the same force. We can communicate motion by elasticity. For instance, if we place a number of coins upon a table touching each other and in a straight line, and strike the last coin of the line by pushing another sharply against it, the piece at the opposite extremity will slip out of its place from the effect of the shock transmitted by the coin at the other end (fig. 29). [Illustration: Fig. 30.—Experiment to illustrate inertia.] When two forces act upon a body at the same time, it takes a direction intermediate. This is known as the resultant. The enormous forces exercised by the heavenly bodies will be treated of later. We will first consider _Inertia_. There are several experiments relating to the subject of Inertia which may be performed. I once witnessed one quite accidentally when taking a walk. [Illustration: Fig. 31.—Another experiment on the same subject.]