LIGHT AND HEAT 241 



Black Body Badiations and Electeon Atmospheees 

 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 large 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 produc- 

 tion 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 distri- 

 bution of energy throughout the various wave lengths is practically 

 independent of the kind of matter, but depends only upon the tempera- 

 ture 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. 



Selective Badiation and Absoeption 



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 radi- 

 ation 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. 



An Ideal of the Illuminating Engineee 

 The subject of selective emission and absorption is one of prime 

 importance to the illuminating engineer. The rods and cones of the 

 VOL. lxxxiii. — 17. 



