Theory of the Chemical Elements. 881 



precise value cannot be given until the nature of its a radia- 

 tion is established. Without entering into a discussion at 

 this point, the opinion may be expressed that these a rays 

 consist perhaps of Nu 2 , with atomic weight 13*25, which could 

 easily be mistaken, except spectroscopically, for helium. 

 There is some reason to believe that this gas may be respon- 

 sible for the three spectral lines mentioned in an earlier 

 section. 



Radium Emanation or Niton. — In discussing a probable 

 constitution of niton, we shall suppose that it is derived from 

 radium by the expulsion of an atom of helium. On this 

 point possible doubt has already been expressed, for reasons 

 only partially indicated, though the indications given show 

 that the a. particles from radium and its products cannot all 

 consist of helium. The value of the atomic weight of lead 

 is another indication. The paper of Rutherford and Royds 

 on the collection of the particles in a vessel may prove the 

 matter in this special case. But if we adopt the usual view 

 as a working hypothesis, the atomic weight of the emanation 

 would be 222*4, if the atomic weight of radium is 22li*4 us 

 determined by Mine. Curie. The nearest value which can 

 be obtained on the present scheme is given by 



Ra Em = 2{2Nu 4 (PfH) 3 . 3Ue 4 (PfH) 



with an atomic weight 222*81. It can be arranged in other 

 forms, one of which might be the thorium emanation, and 

 is, in itself, quite consonant with the scheme, preserving the 

 double and quintuple groupings. It still does so if arranged 

 as 



Ra Em = 2{He 4 (P£H) 3 . 4He 2 Nu 2 (P£fl) 3 }. 



Until further evidence concerning the a. particle is available, 

 this formula, in one form or another, will be adopted. If 

 the atomic weight of helium be added, the atomic weight of 

 radium becomes 226'80. This is very close to the value 

 given by Thorpe's experiments, as described in a recent 

 Bakerian Lecture. The formula is well in line with the 

 scheme, but it does not exhibit radium as a transformation 

 product of uranium if only helium is removed in the form 

 of a particles. 



If this value be correct, it reduces the discrepancy between 

 the aiomic weight difference of radium and lead, and the 

 weight of the five helium atoms supposed to be ejected in a 

 final transformation from radium to lead. The atomic 

 weight of lead is given as 207*1, and with the value 226*80 

 for radium, the five a particles should weigh 19*70, which 



