+----------+------------+------------+---------------+-----------+---------------+---------------+-------------+------------------------+ | 86 Niton |87 -- | 88 Radium | 89 Actinium | 90 Thorium|91 Protactinium| 92 Uranium | | | | Ni 222·0 | | Ra 226·0 | Ac? | Th 232·1 | Pa? | U 238 | | | +----------+------------+------------+---------------+-----------+---------------+---------------+-------------+------------------------+ The Molecular Theory of Physics. From a consideration of the chemical properties of the elements we shall now turn to an examination of the physical characteristics, although in a certain sense chemistry itself is but one special phase of physics. If matter is really constructed of independently existing particles—atoms and molecules—the interplay of the individual parts must determine not only the chemical activities, but also the other properties of matter. Since most of these properties are different for different substances, or in other words are “molecular properties,” it is reasonable to suppose that in many cases explanations can be more readily given by considering the molecules as the fundamental parts. It is natural that the first attempts to develop a molecular theory concerned gases, for their physical properties are much simpler than those of liquids or solids. This simplicity is indeed easily understood on the molecular theory. When a liquid by evaporation is transformed into a gas, the same weight of the element has a volume several hundred times greater than before. The molecules, packed together tightly in the liquid, in the gas are separated from each other and can move freely without influencing each other appreciably. When two of them come very close to each other, mutually repulsive forces will arise to prevent collision. Since it must be assumed that in such a “collision” the individual molecules do not change, they can then to a certain extent be considered as elastic bodies, spheres for instance.