280 wien: recent theories of heat and radiation 



sary to suggest a proper cause for increasing the irregular motions 

 by the amount of the pressure. For the calculation of these 

 irregularities it is altogether probable that the mirror cannot be 

 considered to be a continuous body and the molecular structure 

 of the mirror must be taken into consideration. 



The theory of the Rontgen rays, is also connected in some way 

 with the theory of quanta but is founded on purely electromag- 

 netic considerations. Sometime ago I tried to calculate the wave 

 length of the Rontgen rays from the theory of Stokes and Wiech- 

 ert using only the measurements of energy. The electromagnetic 

 theory gives the energy radiated by the retardation of an electron 

 in a path of definite length. The radiated energy increases with 

 the velocity of the electron and diminishes with the length of the 

 path of retardation. The electron on the other hand radiates 

 only in this path and the wave of electromagnetic disturbance is 

 therefore enclosed between two spheres the centers of which lie 

 at the extremities of the path. Hence the wave length can be 

 calculated from the length of the path, that is, from the velocity 

 and the radiated energy. The energy of the electrons can be 

 calculated from the energy of the excited Rontgenrays. One 

 can therefore calculate the wave length of Rontgen rays from the 

 ratio of the energies of electrons and Rontgen rays. The value 

 found in this way is considerably smaller than that found by obser- 

 vations of diffraction. Another way of calculating the wave 

 length of Rontgen rays is given by the theory of quanta. If 

 secondary electrons are excited by Rontgen rays the velocity 

 acquired is much greater than can be explained by the electro- 

 magnetic theory. Only a few atoms, however, radiate secondary 

 electrons. 



For the simplest explanation of this observation the hypothesis 

 is made that the energy of secondary electrons is derived from the 



energy accumulated in the atom and having the mean value ^ . 



At first those atoms will radiate which possess a quantity of energy 

 not very different from hv, and which need only to absorb a small 

 amount of the incident radiation to reach the critical value hv. 

 These atoms will then radiate and the energy of the emerging 



