_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.