In any of the above cases, we may intelligently, and with propriety, substitute the words, "_Molecules, by means of cohesion, resisting, etc._," for the terms above used, "Molecular Attraction, resisting, etc." All Masses of Substance (probably Molecules as well) are capable of _Expansion and Contraction_, both phenomena, in fact, and in degree, resulting from the relation of the Molecules. Contraction is a "crowding together" of the Molecules; Expansion a "getting apart" of them. _Density_: The amount of Substance in relation to a given bulk. _Volume_--the "size" or "bulk" of a body of Substance. _Mass_--Besides being used to designate a "body" of Substance, composed of two or more Molecules, the term "Mass" is used to designate the "total quantity of Substance in a Body." An application of the above terms may be seen in the following illustration: A quart of water occupies a certain space--and has a certain "volume," "mass" and "density." Convert the same "mass" of Water into Steam, and it expands to a "volume" of 1700 times that of Water--but, as no molecules have been added, the "mass" remains the same--but as a quart of Steam weighs 1700 times less than the same "volume" of Water, the "density" of Steam is 1700 times less than that of Water. As the "volume" of a given "mass" increases, the "density" decreases in the same proportion--but the "mass" remains the same. "Mass" therefore has two factors, _i.e._, "Volume" and "Density." The "Density" of a "Mass" is determined by the _weight_ of a certain "Volume" of it. The above consideration of the "Properties" of Substance dealt only with the Molecular Properties, or Physical Properties, as they are sometimes called--that is, with properties depending upon the existence of the Molecules. When we consider the Molecules as being composed of Atoms, and when we consider the processes whereby these Molecules are built up of, or broken down through the separation of Atoms, we come to the subject of Atomic Properties, or Chemical Properties, as they are often called. _The Atomic Properties of Substance_ consist principally in the power and manifestation of Motion, in the direction of combination, separation, and the complex motions resulting from the same. This Motion is manifested by reason of Atomic Attraction, sometimes called "Chemical Affinity," which we shall consider a little later on in the chapter. Atomic Principles, as above mentioned, are best illustrated by a reference to Chemical changes, and we shall now examine the same. And, the better way to consider Chemical Changes is by comparing them with Physical Changes, or Changes of the Molecules.