RECENT ADVANCES IN SCIENCE 629 



electron to whose ejection the emission of K fluorescent radia- 

 tion is the accompaniment is called a K electron, etc. Prof. 

 Barkla summarises the results of energy measurements thus : 

 (1) The energy of primary radiation required to expel a K 

 electron is greater than that required for the expulsion of an 

 L electron by the energy of a quantum of K fluorescent radia- 

 tion. The energy of a quantum of K radiation is therefore the 

 energy required to move an electron from the position and 

 state of a K electron in the atom to the position and state of an 

 L electron. (2) The energy of primary X-radiation absorbed by 

 each atom from which an electron is expelled varies from just 

 less than two quanta of primary radiation to just more than 

 one quantum as the wave length of the prima^ radiation 

 diminishes and the amount of its quantum increases. This 

 indicates therefore that absorption does not take place by 

 quanta, thus supporting the later view of his own theory 

 adopted by Planck, that while emission of radiation is by 

 quanta, absorption is continuous. (3) One quantum of its 

 K fluorescent radiation is emitted for each K electron ejected, 

 and the emission is subsequent to the ejection. 



These experimental results, though they may have to be 

 modified by the result of still further research on a greater 

 number of substances, afford conclusions which are in accord- 

 ance with the quantum theory of radiation as far as character- 

 istic radiation is concerned, and they offer some support to the 

 theory of atom structure put forward by Bohr. Bohr regards 

 electrons in an atom as possessing certain stable orbits round 

 which they travel with a definite amount of energy subject to 

 ordinary dynamical laws. The electrons, however, have the 

 property of flying from one orbit to another or right out of the 

 atom owing to the attainment of some critical condition. In 

 flying from an outer to an inner orbit, there is an emission of a 

 quantum of radiation of a definite frequency ; in passing from 

 an inner to an outer orbit there is absorption of energy. Prof. 

 Barkla views the processes of characteristic radiation as, first 

 of all, the ejection of a K electron from its stable orbit owing 

 to absorption of energy from the primary beam, followed by the 

 dropping into its place of an electron from an L electron circuit, 

 or one still further out. It is in the latter occurrence which 

 gives rise to the characteristic radiation, and the energy previ- 

 ously absorbed is now (except for the amount carried away as 



