Cohesion: This term is used to indicate the attraction between Molecules, by which they are combined into Masses or Bodies. Cohesion causes the Molecules to unite and cling together, thus forming the Mass.
Adhesion: This term is used to indicate the attraction between Masses which causes them to "stick together" without a cohesion of their Molecules. Adhesion operates through the adjacent surfaces of the two Masses. It may be considered as a "lesser" form of cohesion.
Chemical Affinity (sometimes called Chemism or Atomic Attraction): This term is used to indicate the attraction between the atoms, by which they combine, unite and cling together, forming the Molecule.
Science has before it the task of naming, and classifying, the attraction between the Corpuscles, by which they combine and form the Atom. But whatever the name, it will be seen that it represents but another manifestation of "Attraction."
Arising from Molecular Attraction, or Cohesion, are several "Properties" peculiar to Masses having Molecules, and resulting from the tendency of the latter to resist separation. We had better consider them briefly, in order to understand the power of Molecular Attraction, and its incidents.
Porosity: That property indicating the distances observed by the Molecules in their relation to each other, which varies in different "kinds" of Substance. All Substance is more or less Porous, that is, has more or less space existing between the Molecules—the degree depends upon the "closeness." Compressibility and Expansibility, sometimes mentioned as "properties," are but results of Porosity.
Elasticity: That property whereby bodies resume their original size and form, after having been compressed, expanded or "bent." The result is caused by the inclination of the molecules to resume their original positions. What is sometimes called "Plasticity" is merely the reverse of Elasticity, and denotes a limited degree of the latter.
Hardness: That condition resulting from Molecular Attraction resisting the forcible entrance and passage of other Substance between the molecules.
Tenacity: That condition resulting from Molecular Attraction resisting the forcible pulling asunder, or tearing apart of the Mass. This condition sometimes is called "Toughness."
Malleability: That condition resulting from Molecular Attraction resisting the forcible separation of the Mass by pounding, hammering or pressure. The resistance is "passive," and consists of the Molecules allowing themselves to assume a spread-out formation, rather than to be forced apart.
Ductility: That condition resulting from Molecular Attraction resisting the forcible separation of the Mass by a "drawing out" process. The resistance is "passive," and consists of the Molecules allowing themselves to be drawn out into a formation of the shape of wire or thread, rather than to be pulled apart.
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.
Some Physical Changes in Substance are brought about by Heat, which tends to separate the molecules, or rather to allow them to spread out away from each other, so long as the high temperature is maintained, the degree of their nearness being influenced by temperature. Other Physical Changes are produced by outside Forces separating the molecules to such an extent—to such a distance—that their cohesive force is lost, and the Solid matter is said to be "broken," or even reduced to dust. Other physical changes are brought about by Electricity, causing the Molecules to separate and disintegrate.
Chemical Changes, as distinguished from Physical Changes, do not involve or deal with Molecules, the action being solely upon the Atoms of which the Molecules are composed. Physical Changes separate Molecules from each other, while Chemical Changes destroy and break up the Molecule, so that its identity is forever lost, its Atoms thereafter either existing free from combinations, or else recombining with other Atoms, and forming new combinations. Chemical changes are occasioned by either physical or chemical agencies. The physical agencies generally employed are heat, electricity, light, pressure, percussion, etc. The principle of Chemical Changes is that the Atoms are possessed of, and subject to, what is called "Atomic Attraction" or "Chemical Affinity," which may be defined as an attraction or "love" existing in varying degrees between Atoms. This Affinity causes Atoms of one element to seek out and ally themselves to Atoms of another element, the element of "choice" or "preference" being strikingly in evidence.
Atoms of different elements form marriages, and cling together in harmony, until, perchance, by some physical or chemical agency, the Molecule is brought in sufficiently close connection with another Molecule composed of different elemental atoms, when, alas! one of the Atoms of our Molecule finds that it has a greater Affinity for some other elemental Atom in the second Molecule, and lo! it flies away, leaving its first partner, and seeking the new charmer. Divorce and re-marriage is a common thing in the world of Atoms—in fact, Chemistry is based upon these qualities.
Physical and Chemical Changes gradually transform solid rock to "earth" or "soil." Disintegration, by the action of changes in temperature, rains and atmospheric influences, and other Physical Changes, have slowly worn down the rocks into "dirt," gravel, clay, loam, etc. And Decomposition by Chemical Change that set the atoms free from their combinations has aided in the work.
There is no rest in the world of Substance. Everything is changing—constantly changing. Old forms give way to new, and these, grown old while being born, are, in turn replaced by still newer. And on, endlessly. Nothing persists but change. And yet nothing is destroyed, although countless forms and shapes have succeeded each other. Substance is always there, undisturbed and unaffected by the varieties of forms it is compelled to undergo. Masses may change—and do change. Molecules may change—and do change.