Gravitation: This term is usually applied to the attraction between Masses of Substance, such as the Sun, the Earth, and Masses of Substance on or about the Earth's surface. However, Newton, who discovered the facts of Gravitation, states the Law, as: "Every particle of matter in the Universe, attracts every other particle," etc.
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."