Page:Elementary Text-book of Physics (Anthony, 1897).djvu/100

 what precedes, it is necessary to assume the existence of certain forces other than the mass attraction considered in § 58, acting between the molecules of matter. These forces seem to act only within very small or insensible distances, and vary with the character of the molecule. They are hence called molecular forces. In liquids and solids there must be a force of the nature of attraction holding the molecules together, and a force equivalent to repulsion preventing actual contact. The attractive force is called cohesion when it unites molecules of the same kind, and adhesion when it unites molecules of different kinds. The repulsive force is probably a manifestation of that motion of the molecules which constitutes heat. In gases this motion is so great as to carry the molecules beyond the limit of their mutual molecular attractions: thus the apparent repulsion prevails, and the gas only ceases expanding when this repulsion is balanced by other forces.

72. Structure of the Molecule.—The facts brought to light in the study of crystals compel us to ascribe a structural form to the molecule, determining special points of application for the molecular forces. From this results the arrangement of molecules which have the requisite freedom of motion into regular crystalline forms.

73. Nature of the Atom.—The atom, or the least part into which matter can be divided by any means now known, must itself possess inertia and impenetrability. Our inability to divide the atom, and the demonstration by Lavoisier and others that none of the matter which takes part in a chemical change is destroyed by that change, lead us to assert that the atom is also indestructible. The kinetic theory of heat requires the additional assumption that the atom is generally in motion; and the existence of molecular forces and of chemical combination lead us to assert also that the atoms exert force on one another. These properties were summed up by Newton, who first gave a description of the atom, in a form suitable for use in physical science, in the following words: "It seems probable to me that God in the beginning formed matter in solid, massy, hard, impenetrable, movable