The subsequent difficulties raised respecting the solidity of Matter may be explained thus. And for convenience sake, we will limit the term Matter to such substances as are object to the physical sense, like granite, while Force shall be used to comprise those finer substances, like the Ether, which are impalpable to the physical sense. Matter is composed of very minute ultimate particles which do not touch, but which are held together by Force. The space between the atoms, which would otherwise be _in vacuo_, is _full_ of Force. We might be more exhaustive in our analysis, and say--which would be true--that a space-filling force composes the Universe; and that Matter is only Force in one of its modifications. But without this the other statement is sufficient. When, then, a portion of matter is compressed, the force which holds the ultimate particles in their places is overcome by an external force, and these particles are brought nearer together. Now, how is it with the moving body and the collision? Bisect a line and see the truth. C A--------B 1 A body with a mass of 4 is moving with a velocity of 4 along the line from A to B. At C it meets another body with a mass of 4 at rest. From thence the two move on towards B with a velocity of 2. What has happened? In the body there was a certain amount of force, which set it in motion and kept it in motion. And just here let us make a point. _No force is ever lost or destroyed. It is only transferred._ When a bullet is fired from a gun, it possesses at one _point_ a maximum of force. From that point this force is steadily _transferred_ to the air and other substances, until all that it received from the powder is spent. But at any one point in its flight, the sum of the force which has been transferred since the maximum, and of the force yet to be transferred, will always equal the maximum. Now, how is it respecting the question raised by Mr. Spencer? The instant of contact is a point in time, _not a period_, and the transfer of force is instantaneous. C, then, is a _point_, not a period, and the velocity on the one side is 4 and the other side 2, while the momentum or force is exactly equal throughout the line. If it is said that this proves that a body can pass from one velocity to another without passing through the intermediate velocities, we cannot help it. The above are the facts, and they give the truth. The following sentence of Mr. Spencer is, at least, careless. "For when, of two such units, one moving at velocity 4 strikes another at rest, the striking unit must have its velocity 4 instantaneously reduced to velocity 2; must pass from velocity 4 to velocity 2 without any lapse of time, and without passing through intermediate velocities; must be moving with velocities 4 and 2 at the same instant, which is impossible." If there is any sense in the remark, "instantaneously" must mean a _point_ of time _without period_. For, if any period is allowed,