431 Prof. W. H. Bragg on the Properties and 



however, that there may be a differential effect in respect to 

 heavy and light atoms, as in the case of the X rays, The 

 /3 rays will produce 8 rays in their turn ; and if, as is probably 

 the case, the 7 rays are themselves able to ionize, the product 

 will consist of 8 rays, a conclusion which may be safely 

 adopted from the analogies of the cathode rays on the one 

 hand and the X rays and ultra-violet light on the other. As 

 in the case of the hard X rays, the existence of 7 rays is 

 often made clear by the secondary effects which they produce, 

 as has been shown by Becquerel. 



To sum up what has been said, the ionization which we 

 measure in the ionization-chamber is almost wholly due to 

 the emission of slow-speed electrons from the atoms of the 

 gas contained in the chamber, or of the chamber-walls ; and 

 this is true for all forms of radiation. 



Moreover, there is some evidence to show that the speed 

 of the 8 rays is almost independent of the cause and manner 

 of their production. As has already been said, Fuchtbauer 

 found the velocity of the 8 rays, caused by canal rays, to be 

 about 3 3x10 s , and the same in the case of cathode rays. 

 Logeman found the velocity of the 8 rays, emitted from a 

 plate struck by a. rays, to be such that they were deflected by 

 a weak magnetic field. Ewers found (Phys. Zeit. March 

 1906) the 8 rays of polonium to possess a speed of 3'25 x 10 8 . 

 With these may be compared Lenard^s estimate, viz. 10 8 , of 

 the speed with which the ions leave a plate struck by ultra- 

 violet light. It seems probable that we have here a critical 

 speed for the electron. Below this, it is not able to leave the 

 parent atom. If its velocity exceeds the critical amount it 

 possesses powers of penetration and of causing ionization, 

 the extent of these powers depending on the excess. 



The existence of a common speed for all 8 rays may, of 

 course, imply that the ejection is not directly effected by the 

 ionizing agent, but that the latter simply precipitates the 

 discharge. A man running through a battery might pull 

 the triggers of some or all of the guns which it contained, 

 and the velocity of the shot would not depend on the strength 

 of the man, nor the rate at which he ran, nor how much 

 energy he spent in the transit. And so it may be understood 

 why 8 rays are projected at a speed which is independent 

 of the nature of the agent, as has been said above. So also 

 it appears to be independent of the. intensity of the agent's 

 action. Fuchtbauer found the velocity of the 8 rays produced 

 by canal rays to be independent of the intensity of the 

 primary rays : Lenard found the same for ultra-violet light. 



