536 On Ionization by Cumulative Action. 



conditions become more favourable to cumulative action and 

 less favourable to direct ionization by single impacts. We 

 should expect, therefore, that cumulative action would be 

 particularly important in the temperature ionization o£ gases, 

 and therefore that the part played by radiation in promoting 

 temperature ionization would be very important. 



An illustration of the possible importance of such a 

 concept of temperature ionization is seen in the opportunity 

 which it affords of explaining certain failures of the exceed- 

 ingly interesting and important theory of Ionization in the 

 Solar Chromosphere, which has recently been proposed by 

 Dr. Megh Nad Saha *. He applies Nernst's equation of the 

 reaction isobar to the calculation of the percentages of 

 ionization of different elements, by considering ionization to 

 be a dissociation with heat of dissociation measured by the 

 ionizing potential, and by assuming the element to be in an 

 enclosure at the temperature T of the Sun's chromosphere. 

 This is equivalent to assuming that the element is subjected 

 to molecular bombardments and black body radiation 

 characteristic of the temperature T. This le'ads to the con- 

 clusion that the degree of ionization of an element at anv 

 pressure depends only on its ionization potential and the 

 temperature T. The theory thus developed accounts beauti- 

 fully for numerous characteristics of the solar spectrum. It 

 has been pointed out by Professor Hussell, however, that 

 there are several instances in which Snha's theory seems 

 inadequate t. For instance, the ionization potentials of 

 barium and sodium are practically equal, yet barium is 

 apparently completely ionized in the chromosphere, whereas 

 sodium is not, as evidenced by the absence of all except the 

 enhanced lines of barium, which are strong, while the 

 sodium D lines prove an abundance of un-ionized sodium. 



This may be accounted for if the influence of radiation in 

 promoting ionization be considered, as will be pointed out 

 in a Note in the Astrophysical Journal by Professor Russell 

 and the writer. Whereas the energy of molecular bombard- 

 ments is characteristic of the temperature T, the spectral 

 distribution of radiant energy is not, since it is present as an 

 outward flux, from the hotter interior through the selectively 

 absorbing photosphere. Various factors, such as abundance, 

 atomic weight, and chemical affinity, may determine the 

 extent to which any element in the chromosphere is shielded 

 from radiation of its own resonance type by the " blanketing " 



* Phil. Mag. xl. pp. 472, 809 (1920). 

 t Astrophys. Jour., in print. 



