11G Prof. E. Hutherford on the Velocity and 



it is to be expected that the period of transformation of 

 radium C should be less than one minute, while that of 

 thorium X or actinium X should not be more than afew seconds- 

 There is a possibility that further investigation may yet 

 remove these products from the list of exceptions. It is 

 possible that two successive changes may occur, for example, 

 in radium C, the one change a rayless one of period ID- 

 minutes, and other a ray change of very short period. A. 

 similar result may hold for thorium X and actinium X. 

 Such possible double changes, where one of the changes is- 

 very rapid compared with the other, would be extremely 

 difficult to detect. There is certainly no evidence so far 

 obtained which in any way negatives such a possibility. 



It will be seen that, in most cases, the period of transfor- 

 mation decreases very rapidly for a slight increase in velocity 

 of the expelled a. particles. I have not succeeded in finding 

 any simple connexion between the velocity of expulsion of 

 the gl particle and the period of transformation of the product. 

 Halm has already drawn attention to the interesting fact 

 that the corresponding products of thorium and actinium 

 occupy the same relative position when the products of each 

 element are arranged in order of increasing velocity of the 

 a particles expelled from them (see previous table). 



Rate of emission of energy. 



We have seen that the energy of the a. particles due to the- 

 successive disintegrations of one atom of the parent element 

 is 12*5 X 10 u . e ergs for radium (excluding radium F), 

 17*7 xl0 14 .<? for thorium, and 14*0 x 10 14 . e for actinium. 

 In a previous paper I have shown that the heating effect of 

 radium is, in great part, a measure of the kinetic energy of 

 the expelled a, particles. We may consequently assume with 

 confidence that thorium, uranium, and actinium also emit 

 heat energy in amount proportional to the kinetic energy of 

 the expelled a particles. Until the number of atoms which 

 break up per second is determined accurately for each of 

 these elements, the actual heat emission cannot be calculated 

 with cef taiiity. From measurements of the relative activity 

 of thorium and uranium compared with radium, it can, 

 however, be deduced that the heat emission per gram of" 

 radium is about tw r o million times as great as the corresponding- 

 value for uranium and thorium. 



Since the heat emission of radioactive substances is a 

 measure of the energy of the expelled a. particles, radioactive 

 matter however sparsely distributed will emit heat energy 



