2 SCIENCE PROGRESS 



the HiK lines are enhanced lines and leading members of the 

 principal pair-series, whilst the g line, of the normal type, is 

 the first member of the system of single lines. Now Fowler 

 has shown that the spectroscopic constant of the series formula 

 of the double lines is 4N, instead of the usual Rydberg number 

 N, which is that of the single-line formula. On Bohr's theory 

 of the atom it follows that the radiating system has effectively 

 a double charge, so that the system has been produced by the 

 removal of one electron from the normal atom. 



Assuming this theory, the highest level of the chromosphere 

 would require to be the region of the most intense ionisation. 

 The quantitative application to the solar chromosphere has 

 been worked out recently by M. N. Saha {Phil. Mag., 40, 472 and 

 809, 1920), using a method developed by Eggert {Phys. Zeit- 

 schrifi, December 19 19), based upon Nernst's " Reaction-isobar." 

 The ionisation of a calcium atom may be regarded as taking place 

 according to the scheme : 



Ca 5 Ca + + e - U 



where Ca denotes a normal atom of calcium vapour, Ca+ an 

 atom which has lost one electron, and U the quantity of energy 

 liberated in the process. The latter can be calculated from the 

 value of the ionisation potential V of the element, if known, 

 for the amount of energy required to detach one electron from 

 the atomic system is eV/300, V being measured in volts. If 

 U refers to one gm-atom and N is Avogadro's number, J the 

 mechanical equivalent of heat, then 



U = eV . N/J . 300 = 2*302 X lo^V calories. 



The thermodynamical equation of gaseous equilibrium is 

 applied to the above reaction. Cp denoting the specific heat 

 at constant pressure, Saha assumes that (Cp)ca = (Cp)ca+ and 



that (Cp)e = - R, the electron being supposed to behave like 



a monatomic gas. The chemical constant of the electron is 

 calculated from the Sacker-Tetrode relation, taking for the 

 atomic weight of the electron 1/1836, and is found to be — 6-5. 

 In this way the equation of the reaction-isobar is found to be : 



X* U 



log P = ^ + 2'5 log T —6-5 



^ I -X' 4-571 T ^ ^ 



In this formula x denotes the fraction of the calcium atoms 

 which are ionised, P the total pressure, and T the absolute 

 temperature. The formula makes it evident to what a large 

 extent the degree of the ionisation is dependent upon the pres- 

 sure, a reduction in the pressure greatly increasing the ionisation. 

 It is now possible to calculate the degree of ionisation 



