496 



Sir E. Rutherford on the 



emission of energy by supposing it to have its origin in the 

 electronic distribution. It seems more likely that a very 

 high speed electron is liberated from the nucleus, and in its 

 escape from the atom sets the electronic distribution in 

 violent vibration, giving rise to intense y rays and also 

 to secondary (3 particles. The general evidence certainly 

 indicates that many of the high speed electrons from radio- 

 active matter are liberated from the electronic distribution 

 in consequence of the disturbance due to the primary electron 

 escaping from the nucleus. 



Charge on the Nucleus. 



"We have seen that from an examination of the scattering 

 of a. particles by matter, it has been found that the positive- 

 charge on the nucleus is approximately equal to \Ke, when 

 A is the atomic weight and e the unit charge. This is» 

 equivalent to the statement that the number of electrons in 

 the external distribution is about half the atomic weight in 

 terms of hydrogen. It is of interest to note that this is the 

 value deduced by Barkla * from entirely different evidence, 

 viz. the scattering of X rays in their passage through matter.. 

 This is founded on the theory of scattering given by 

 Sir J. J. Thomson, which supposes that each electron in an 

 atom scatters as an independent unit. It seems improbable 

 that the electrons within the nucleus would contribute to> 

 this scattering, for they are packed together with positive 

 nuclei and must be held in equilibrium by forces of a dif- 

 ferent order of magnitude from those which bind the external 

 electrons. 



It is obvious from the consideration of the cases of 

 hydrogen and helium, where hydrogen has one electron and 

 helium two, that the number of electrons cannot be exactly 

 half the atomic weight in all cases. This has led to an 

 interesting suggestion by van den Broekf that the number 

 of units of charge on the nucleus, and consequently the 

 number of external electrons, may be equal to the number of" 

 the elements when arranged in order of increasing atomic 

 weight. On this view, the nucleus charges of hydrogen r 

 helium, and carbon are 1, 2, 6 respectively, and so on for 

 the other elements, provided there is no gap due to a mis?ing 

 element. This view has been taken by Bohr in his theory of 

 the constitution of simple atoms and molecules* 



Recently strong evidence of two distinct kinds has been 



* Barkla, Phil. Mag. xxi. p. 648 (191R 



t van den Broek, Phys. Zeit. xiv. p. 32(1913). 



