184 THE ROYAL SOCIETY OF CANADA 



We may account for the absence of rays of this series as follows: 

 Radium B, Radium C, and radium D emit different types of 7 rays, 

 although radium B and radium D have the same atomic number 82, and 

 radium C the atomic number 83. To explain these differences, Ruther- 

 ford assumes that a /3 ray ejected from the nucleus of a radioactive 

 atom always moves in a particular direction with respect to the atom, 

 and therefore may excite one type of characteristic radiation on one 

 product, whereas in another product, it may excite quite a different 

 type. If this is the case, it may be that the probability of a ray of 

 radium E entering a lead atom in the proper direction to excite the 

 7 rays of the series H is too small for this type of 7 rays to be de- 

 tected. Further, there is a possibility that the 7 rays for radium C 

 are excited in the nucleus and not in the electronic rings surrounding 

 the nucleus, in which case the probability of the 7 rays being formed 

 by j8 rays entering the atom would be very small. In any case it 

 will be noticed that the /3 rays of radium E, with rays corresponding 

 to potentials of 150,000 to 600,000 volts, excite 7 rays corresponding 

 to /3 rays with a potential drop of less than 177,000 volts. The 

 chance of a fast ray being stopped in an atom, sharply enough to 

 give up all its energy in the form of 7 radiation seems very small. The 

 fastest rays with which one is ever likely to experiment are those of 

 radium C and thorium C, some of which correspond to a potential 

 drop of over 2,000,000 volts, and it may be noted that the 7 rays 

 excited in lead by the /3 rays of radium C 1 are less penetrating on the 

 whole, than the rays of radium C, although there may be more pene- 

 trating rays present. It seems quite possible that the most penetra- 

 ting 7 rays that are excited by /3 rays may not depend so much on the 

 speed of the rays as on the structure of the atom in which the 7 ray 

 is excited. In other words there may be a limit to the penetrating 

 power of 7 rays formed by /3 rays in any particular element. 



Summary. 



1. An account is given of the work of Rutherford and Andrade 

 which has led them to the conclusion thai two of the types of 7 rays 

 emitted by Ra B belong to the K and L series of characteristic radia- 

 tions, while the 7 rays of radium C belong to yet another series, 

 which they have called the H series. 



2. An account is given of an experiment by the writer on the 

 7 rays excited in lead by the 13 rays of radium E, which shows that 

 rays of the H series are not excited to any extent by these fast rays, 

 although they have sufficient energy. 



•Chadwick, Phil Mag. 24, p. 594, 1912. 



