And in this consideration, let us not forget the important part that Gravitation--that most wonderful of all Forces--plays in the grand scheme of Nature. Not only does this Force cause the planets to circle around the sun, and, perhaps that sun around another sun, and so on, and on until the matter becomes unthinkable--not only this, but it performs a million parts in the affair of earthly Matter, as we shall see in a later chapter. The Force of Gravitation is one of the greatest mysteries confronting Science to-day, although many believe it a simple question. Gravitation and the Universal Ether contain the great secrets of Nature that Man is striving to unveil. And yet, so "common" is Gravitation that the race, including almost all the scientists, take it as a "matter of course." We shall devote much attention to the question of Gravitation in the forthcoming chapters of this book, for it plays a very important part in the general theory of Dynamic Thought, upon which this book is based. We shall have a special chapter devoted to it, a little later on, and the matter will also come up for explanation further on in the book. But, in the meantime, let us consider the other forms of Energy, _viz._, Heat, Light, Magnetism and Electricity, which with Gravitation and Attraction of other kinds, form the Forces of Nature. CHAPTER IX RADIANT ENERGY The "kinds" of Energy are very few, although the methods of using, applying and manifesting same are innumerable. Let us begin with one of the best known forms of Energy, namely, Heat. _Heat_ was formerly regarded as a very fine fluid or substance, called "caloric," which was supposed to enter into Substance and then manifest the phenomenon of "heat." This idea has long since been relegated to the scrap pile of Science. The present theory, which is supported by a mass of evidence obtained through investigation and experimentation, is that Heat is a form of Energy, arising from the vibratory motions of the Particles of Substance--a "Mode of Motion." The degrees of Heat are termed "Temperature." Temperature depends upon the rate of the heat-vibrations of the Particles of Substance, either arising from the Original Motion of the Particles, or else from vibrations or Motion aroused in them by transmission from Particles of other bodies of Substance--these vibrations being "contagious." Temperature then means "the measure of the vibrations of the Particles." All bodies of Substance have _some_ degree of Temperature--some degree of heat-vibration of its Particles. Science has a pleasant "scientific friction" of an Absolute Zero at the degree of 491 below Zero, Fahrenheit, but this is merely an imaginary something with which the grown up children of Science amuse themselves.