Molecular Motion
Perhaps to a primitive mind the earth consists of stones, grains of sand, dust particles, drops of water, and similar things. In other words, it is made up of parts. One cannot contemplate Nature for long without noting that disintegration and transformation are continually going on. Ice melts to water; the water vaporizes; the vapor condenses again to water; it freezes to ice. Clouds dissipate themselves in rain or snow. Rock disintegrates into sand; the sand into dust; the dust is carried away by wind and water. Thus, visible things become invisible and the invisible may become visible.
It is not surprising that Anaxagoras (500-428 B.C.) propounded the principle that the material of Nature was compounded of and could be resolved into elementary seeds of matter. This idea found advocates throughout the ages, although it was little more than a speculation in early centuries. Leucippus believed that the universe consisted of limitless empty space and of matter, the latter consisting of numberless, indivisible atoms. He also thought that atoms were always in motion. As usual, there were many fantastic ideas interwoven in these speculations.
Democritus (460-370 BC), a pupil of Leucippus, developed the theories of his teacher to such an extent that generally his name is the only one associated with these early speculations of empty space and atoms as elements of the cosmos. He also developed the idea that all the material of the universe was produced by the motions of these atoms. He was an extreme sceptic as indicated by the remark attributed to him, “We know nothing; not even if there is anything to know.”
Although many of his speculations can be very easily modified to fit the facts of the physical world as we know it today, he contributed little of real note to science excepting an example of the value of a critical attitude. Nevertheless, the idea of atomism had to have a beginning and it matters not if it began in metaphysics and alchemy. It has eventually arrived a definite reality. It aided in the development of chemistry which in turn developed atomism. Spectroscopy did much to place the atomic theory of matter on a firm foundation. Finally radio-activity, that marvelous exhibition of the birth of one element from another, has made us even better acquainted with the atom.
In the development of the atomic theory of matter, the foundational laws of the great stellar world were applied to the tiny worlds far beyond the revealing power of the microscope. The great laws which were developed to explain the motions of the Moon, planets, and other celestial bodies and were so clarified by Newton, soon began to serve to explain the structure and properties of matter and eventually of atoms themselves. Motions and masses were attributed to minute molecules, atoms, and electrons with the result that the energy of motion of these small particles became a very important factor in explaining various physical phenomena. Hence, the introduction of the term, kinetic energy. As will be shown later, modern developments impose limitations upon the applications of Newtonian mechanics, but for the present we need not consider them in detail.