Inertia is possessed quite as much by a moving body as a body at rest. The definition given points this out, as it states that matter cannot of itself change its state of motion. If a body therefore is in motion, it requires a certain amount of resistance to bring the body to a state of rest, or the loss of an equal amount of energy, by friction or otherwise, equal to the quantity which it absorbed in order for it to be set in motion. We get numerous examples of this property of the inertia of bodies in our daily experience. Many of the accidents that befall people in various ways are due to this property of the inertia of matter. A cyclist is riding a machine down-hill, and loses control over his machine, with the result that he runs into a wall, and is killed. Now what has happened? The cyclist has participated in the motion of the machine, with the result that when the machine has been suddenly stopped, the body has been thrown forward owing to the momentum it had acquired. We are constantly being affected by the property of inertia of matter, in tram and train and bus. Whenever any of these are suddenly stopped, or suddenly started, we are thrown either backward or forward, owing to the body either not having acquired the motion of the train, or, having acquired it, is unable to lose its motion as quickly as the train, and is therefore thrown forward. CHAPTER IV AETHER IS MATTER ART. 42. _Aether is Matter._--The hypothesis of an Aether which fills all space was made in order that scientists might be able to account for certain phenomena of Light, which otherwise were difficult to account for. Its existence is demanded not only for the phenomena of Light, and Heat, but, in view of the comparatively recent researches of Hertz on "Electric Waves," of Electricity also. The Aetherial Medium is generally assumed to be that fundamental medium, by means of which possibly all the properties of matter, and all the phenomena of motion of the universe are to be explained. Light and Heat have been proved to be due to the periodic wave-motion of this universal Aether, while from the investigations and researches of such men as Clerk Maxwell, Poynting, Thompson and Hertz, it has been proved that electro-magnetic phenomena are due to this same medium. Several different forms of Aether have been postulated by various philosophers from time to time, but the only Aether that has survived, is that which was first conceived by Huyghens to explain the phenomena of Light, though it was Thomas Young who finally succeeded in placing the conception of the Aether on a sound basis. Each discovery of science has only strengthened the hypothesis and existence of the Aether, the latest discovery, that of wireless telegraphy so successfully developed by Signor Marconi, being attributed to the electro-magnetic properties of this self-same Aether.