Page:Popular Science Monthly Volume 83.djvu/245

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Since the advent of the theories of liquid and gaseous ionization, many attempts have been made to construct a system composed of ions and electrons of various kinds that would be capable of explaining the phenomena of optics and radiant heat. In the case of black body or pure temperature radiations, the theory has been quite successful and seems to correctly describe the actual conditions. Solids or liquids are known to contain largo numbers of electrons and when these bodies conduct metallically there is good reason to believe that the electrons move about in these bodies like gaseous molecules in a gas, the law of the equipartition of energy applying to an electron "gas" in a metal in the same way as it does to gases outside the metal. The emission of light and heat under these conditions is presumably due to the production of electromagnetic waves when the electrons are greatly accelerated or retarded in their motion. Laws of radiation like those of Wien and Planck can be derived from the conditions that would be expected to hold in an electron atmosphere. In this type of radiation the distribution of energy throughout the various wave lengths is practically independent of the kind of matter, but depends only upon the temperature and the nature of the electron atmosphere. Thus the radiation constants are universal constants depending upon one kind of radiating and absorbing system, the electron.

Many sources of light and radiant heat emit radiations whose energy distribution over the various wave lengths is very different from that of a black body radiation. These radiations are selective and depend upon the nature of the body that is emitting or absorbing. Emission spectra illustrating this selective radiation are spark, arc, band and other spectra. Colored objects all show selective absorption. The problem of unraveling the constitution of the centers of selective radiation and absorption is a very difficult one and at present many efforts are being made to correlate the possible constitution of such centers with the ordinary molecular, atomic and ionic theories of matter. During recent years the trend of theory has been largely directed towards the view that emission and absorption spectra originate in systems that have a more or less momentary existence, owing to the fact that such optical systems are essentially dynamic in nature. It is very natural, therefore, that especial efforts should be made to find the existence of these momentary systems during periods of ionization and recombination of atoms, molecules, ions and electrons.

The subject of selective emission and absorption is one of prime importance to the illuminating engineer. The rods and cones of the