382 A CENTUEY OF SCIENCE 



the laws which govern the motions of electrons and pos- 

 itively charged particles inside the atom. 



Black Radiation. The significance of the problem was 

 first brought to light through the study of black radia- 

 tion. By a black body is meant one whose distinguishing 

 characteristic is that it emits and absorbs radiation of all 

 frequencies, and black radiation is that which will exist in 

 thermal equilibrium with such a body. The interest of 

 this type of radiation lies in the fact, demonstrated by 

 Kirchhoff, that its nature depends only upon the temper- 

 ature of the black body with which it is in equilibrium, 

 and on none of this body's physical or chemical charac- 

 teristics. Thus we may speak of the " temperature " of 

 the radiation itself, meaning by this the temperature of 

 the material body with which it would be in equilibrium. 



The problem of black radiation is to find the distribu- 

 tion of energy among the waves of different frequencies 

 at any given temperature. The first step toward a solu- 

 tion was made when Stefan showed experimentally, and 

 Boltzmann as a deduction from thermodynamics and 

 electrodynamics, that the total energy density summed 

 up over all wave lengths varies with the fourth power of 

 the absolute temperature. If the energy density is 

 plotted as ordinate against the wave length as abscissa, 

 the experimental curve for any one temperature rises 

 from the axis of abscissas at the origin, reaches a maxi- 

 mum, and falls to zero again as the wave length becomes 

 infinitely great. Now Wien's displacement law, the 

 second important step toward the determination of the 

 form of this curve, shows that as the temperature is 

 raised the wave length to which its highest point cor- 

 responds becomes shorter, in fact this particular wave 

 length varies inversely with the absolute temperature. 

 This theoretical conclusion is entirely confirmed by 

 experiment. (J. W. Draper, 4, 388, 1847.) 



Farther than this general thermodynamical princi- 

 ples are unable to go. Statistical mechanics, however, 

 asserts that when a large number of like elements are in 

 thermal equilibrium, the average kinetic energy asso- 

 ciated with each degree of freedom is equal to a universal 

 constant multiplied by the absolute temperature. This 

 " principle of equi-partition of energy " has been applied 



