﻿1132 Prof. M. N. Saha on Temperature Ionization of 



come simultaneously in the same octant, and both would 

 be carried off by an a-particle which chanced to pass close 

 to them. 



The alkaline earths resemble helium in so far that they have 

 two valency electrons in the same part of the atomic volume 

 in the outermost region of the atom *. 



In solutions we have always Oa ++ -atoms and never Ca + - 

 atoms, because the electric forces act equally strongly from 

 all directions. In the cases considered by us, Ca + -atoms are 

 more probable than Ca ++ -atoms, because the ionizing agencies 

 act from one side only. 



The cases of recombination of Ca + and e, Na + and e may 

 be considered in the same light, and we find that in the case 

 of Ca + and e, recombination is more difficult than between 

 Na + and e. If we consider an Na + -atom, we find that there 

 are no electrons in the outermost ring, and the positive lines 

 of force proceeding from the nucleus act equally strongly 

 within the 4-7T solid angles about the atom. From whichever 

 side the electron may approach the Na + -atom, provided 

 other things (energy, distance) are of the right order, the 

 electron will be captured by Na + . Not so in the case of Ca + . 

 It has still got a valency electron in the outermost rings; 

 lines of force proceeding from the nucleus are strongly con- 

 centrated on it. In other words, to use the language of 

 Stark, there is a negative patch on one side. An electron 

 cannot be captured if it approaches Ca + from this side. It 

 can be captured only when it approaches the Ca + -atom 



47T 



within only a definite fraction — of the total solid angle 



j n ° 



about the Ca-atom, where n is a number > 1. We may call 

 4 n ' the " steric factor." 



These considerations show that for an atom like Ca, ioniza- 

 tion is easier, and recombination of Ca + and e is more difficult 

 than recombination of Na + and e. For trivalent and tetra- 

 valent elements like Sc and Si these considerations will apply 

 with even stronger force. 



III. 



It is a more difficult task to take account of the above facts 

 in a statistical theory. To Boltzmann we owe the idea that 

 when two atomic species A and B associate, every case of 

 approach of A and B does not result in a combination, but 

 only when A and B present to each other certain definite 



* It may be pointed out that Langmuir places helium at the head of 

 the alkaline earths (see Loring, ' Atomic Theories '). 



