THE mechanical motions of nature are transmitted by solids and fluids from sources that consist of more or less well known mechanical systems. Waves on a pond may be due to a boat moving over the surface of the water. Sound waves in air may be due to the vibrations of a tuning fork. Wireless telegraph waves may be due to high frequency electromotive force and current waves in electrical circuits. In general the source of the above type of wave motion is a kind of mechanism that can be made in the laboratory or in the shop—a mechanism that is man-made and whose operation is quite obvious to us.
The phenomena of light and radiant heat introduce to us a type of wave motion that is altogether different. Not only may the medium that transmits this wave motion possess entirely different properties from that of matter, but the mechanisms that take part in the emission and the absorption of the wave motion are altogether different from any that we have been able to make in our laboratories. No one has succeeded in producing radiant heat, much less visible light and ultraviolet radiations by means of electromagnetic oscillator^;, although such a feat may be possible.
Inasmuch as matter is the source of all heat and light radiations, the mechanism responsible for the emission and absorption of these radiations must be intimately related to the nature and constitution of matter itself and therefore theories of emission and absorption systems depend to a large extent upon our theories of the nature of atoms and molecules. It must be remembered, however, that the nature and constitution of atoms and molecules that explain chemical and many other phenomena need not necessarily be at all related to the systems taking part in heat and light radiations.In the past many different hypotheses have been advanced to extend the atomic and molecular conceptions of Dalton, Clausius, Maxwell and others. As long as the elastic solid theory of the ether prevailed, it was frequently assumed that the vibrating systems emitting light and radiant heat were of a mechanical nature due to the development of stresses and strains. The electromagnetic theory of Maxwell, while not even suggesting the nature of the mechanism, metamorphosed our views of radiant energy and indicated the whole phenomenon to be an electromagnetic one.