Perhaps it would be as well to begin by asking ourselves the question: "What is Substance?" The answer seems to be: "Anything that takes up room; the Body aspect of Things; matter occupying space, etc." Some writers have spoken of Substance as "something tangible--that can be felt," but this definition will not do, for there are forms of Substance too fine to be felt. And so, perhaps the definition "The Body of Things," is as good a definition as any, taken in connection with the thought that it "takes up room." Science divides Substance (which it calls "Matter") into four general classes, _viz._: (1) _Solid Matter_, which is Substance, the parts of which closely adhere and resist impression, such as stone, wood, flesh, etc., the degrees of solidity varying greatly, and sometimes shading into the next class, which is called: (2) _Liquid Matter_, which may be described as Substance, the parts of which have a free motion among themselves, and easily yield to impression, such as water, molasses, etc., the degree of fluidity ranging from some liquids that flow very slowly, such as hot pitch, up to others that flow very freely, such as water, wine, etc., the property of fluidity being also shared by the next higher class, which is called: (3) _Aeriform Matter_, which is Substance in the form of "elastic fluid," such as air, gas, vapor, etc.; and (4) _Radiant Matter_, which is of recent recognition, and which is an ultra-gaseous form of Substance, utterly unlike anything ever before known, consisting of the tiniest particles of "corpuscles" of Substance finer and more subtle than the rarest form of atomic substance known to Science. The three classes are well represented by (1) Earth (solid); (2) Water (liquid); (3) Air (aeriform); (4) The Corpuscles or Electrons, or particles of electrified substance, first noticed in connection with the X Rays, Radium, etc. But it must be remembered that these four classes of Substance are not fixed or permanent--on the contrary they are changeable either under pressure, when subjected to heat, or under the influence of electricity, etc. In fact the word "condition" is more applicable than the term "class." The condition or class of a particle of Substance may be changed into another class or condition by the application of the agencies above named. The same substance may exist in two or three classes, under different circumstances. Solids may be changed into liquids, and liquids into gases, and _vice versa_. Metals may be melted, then changed into gas, according to the degree of heat applied. Liquids may be changed into vapor by the application of heat, or into solids by the withdrawal of heat.