﻿hy Moving Electrified Particles. 457 



divided into definite units or quanta, the character of the 

 radiation when excited would be independent of the means 

 used to excite it, but it would not be excited unless the 

 velocity of the primary cathode rays exceeded a critical value. 



This radiation has all the characteristics of the " charac- 

 teristic Rontgen radiation " discovered by Barkla. Barkla 

 made the very important discovery that any element whose 

 atomic weight is greater than 39 gives out, when exposed 

 to Rontgen radiation or cathode rays, secondary Rontgen 

 radiation whose type depends only on the nature of the 

 element. For the same element the " characteristic " radiation 

 is the same whatever be the character of the primary radia- 

 tion, with this proviso, that the characteristic radiation is not 

 excited unless the primary Rontgen rays are " harder " than 

 the characteristic radiation, or if the cause of excitation is 

 cathode rays unless these are moving faster than a certain 

 critical velocity ; these are just the characteristics of the 

 radiation emitted during the recombination of the systems 

 of the kind we have been considering. Whiddington has 

 shown that the least velocity of the cathode ray which can 

 excite the characteristic radiation of an atom of atomic 

 weight A is, in the case of all the elements he investigated, 

 approximately equal to 10 8 A. cm. /sec. 



The existence of systems which are first dissociated and 

 then restored to their original condition by the falling in of 

 a corpuscle carrying with it kinetic energy, will also explain 

 the diminution of specific heat with temperature. For if 

 this is the way by which kinetic energy is communicated to 

 the system, it is necessary that the system should first be 

 dissociated. If this is to be done by corpuscles their energy 

 must exceed a certain value w. The kinetic energy of the 

 free or quasi free corpuscles in a body is proportional to the 

 absolute temperature 6, let it equal ad. Then when u6 is < iu 

 the system will not absorb energy and so will not affect the 

 specific heat, but when a6 is > w the system can be dissociated, 

 can therefore absorb energy and therefore increase the specific 

 heat ; the specific heat therefore will be greater when ad> w 

 than when a0 < w. If we calculate on these lines the specific 

 heat of a body whose molecules contain svstems which require 

 for their dissociation iv u w 2i . . . w n units of work, and suppose 

 that the energy is distributed among the corpuscles according 

 to Maxwell's law, we get an expression for the specific heat 

 as a function of temperature of the same form as that given 

 by Einstein, which is in accordance with the remarkable 

 results obtained by Professor N crust and his pupils at very 

 low temperatures. 



