Origin of the Radiation from Hot Bodies. 22$ 



one-thousandth part of the radiant energy appears as light : 

 practically the whole of it is in the radiation whose wave- 

 length is comparable with the thickness of a Rontgen pulse. 



The preceding estimate is based on the assumption that 

 the cathode ray is stopped at the first collision ; i£ it has to 

 make several collisions before losing its energy, the amount 

 of radiant energy emitted by it will be reduced. For suppose 

 instead of being stopped by one impulse P it is stopped after 

 n collisions by n impulses each equal to P/n, then though the 

 number of collisions is increased n times, the amount of 

 energy radiated at each collision, being proportional to the 

 square of the impulse, will be reduced to 1/n 2 of its former 

 value. Thus the amount of energy radiated will be 1/n of 

 that calculated on ths assumption that there was only one 

 impulse. 



The total amount of radiation when 1ST particles moving 

 with a velocity u are stopped is 



'dirYaf 



a being the time taken by a collision. Now we have seen 

 grounds for believing that a is inversely proportional to the 

 square of the velocity of the corpuscles. If this is the case, 

 however, the energy radiated varies as 'Mil 4 ' ; i. <?., the energy 

 in the Rontgen radiation is proportional to the square of the 

 energy in the cathode rays, provided the number of corpuscles 

 striking against the anti-cathode in unit time is constant. I 

 am not aware of any experiments on the connexion between 

 the effect of the velocity of the cathode rays on the proportion 

 between the energy in the Rontgen rays and the cathode rays 

 which give rise to them. 



We have seen, too, that if the Second Law of Thermo- 

 dynamics applies to radiant energy, then a the time of a 

 collision must, when the velocity of the corpuscles remains 

 the same, be the same for all bodies. Now the amount of 

 energy radiated as Rontgen rays when a given stream of 

 cathode rays is stopped depends only upon a ; and hence, if 

 the preceding considerations are correct, ought to be the 

 same whatever may be the material against which the cathode 

 rays strike. We know, however, that far fewer Rontgen 

 rays are produced when the stream of cathode rays falls 

 against an aluminium target, than when they fall upon one 

 made of platinum or lead. It must be remembered, however, 

 that the impact of the cathode rays, in addition to producing- 

 Rontgen rays, produces also secondary cathode rays ; and 

 these secondary rays, both at their starting and afterwards 



