The atoms for which there are enough known lines of different excitation energies in the solar spectrum are those of calcium, chromium, titanium, and iron. The correlation between the excitation potential associated with a given line and the intensity of the line in the solar spectrum is illustrated by the preceding tabulation. Successive columns give the atom, the excitation potential, the [Pg 115] computed fractional concentration, expressed logarithmically, and the observed intensity, taken from Rowland’s table. It will be seen that the correlation is very marked, and that it appears to furnish good evidence that the theory of thermal ionization predicts correctly the relative tendencies of the atoms to absorb the different frequencies. The fractional concentrations are of course not absolute values, as the number of atoms in a state of high excitation is a definite fraction, not of the whole number of atoms, but of the number left over from the lower excitations. Neither are the intensities given by Rowland absolute, and therefore the comparison appears sufficient to show the strong correlation between excitation potential and solar intensity. FOOTNOTES: [370] Proc. Roy. Soc., 99A, 136, 1921. [371] M. N. R. A. S., 83, 403, 1923; ibid., 84, 499, 1924. [372] M. N. R. A. S., 83, 403, 1923. [373] M. N. R. A. S., 84, 499, 1924. [374] R. H. Fowler, Phil. Mag., 45, 1, 1923. [375] Bohr, Mem. Ac. Roy. Den., 4, 2, 76, 1922. [376] Ap. J., 59, 1, 1924. [377] Chapter I, p. 9. [378] Proc. Roy. Soc., 103A, 413, 1923. [379] M. N. R. A. S., 83, 404, 1923. [380] H. C. 258, 1924. [381] H. C. 252, 256, 1924. [382] Phil. Mag., 47, 209, 1924. [383] Chapter I, p. 21. [384] Proc. Phys. Soc. Lond., 36, 94, 924. [385] P. 45. [386] Ap. J., 59, 197, 1924. [387] See, for instance, H. J. H. Fenton, Outlines of Chemistry, 128, 1918. [388] Lindemann, quoted by Milne, Observatory, 44, 264, 1921. [389] Milne, Observatory, 44, 264, 1921. [390] Chapter VI, p. 94. [391] C. R., 171, 1106, 1920. [392] Russell, Ap. J., in press. [393] A. S. King, Mt. W. Contr. 247, 1922. [394] A. S. King, Mt. W. Contr. 233, 1922. [395] Ap. J., 57, 20, 1923. [396] Mt. W. Contr. 225, 1922. [397] Mt. W. Contr. 236, 1922. [398] Saha and Swe, Nature, 115, 377, 1925. [399] Nature, 115, 534, 1925. [400] Phys. Zeit., 18, 121, 1917. [401] Phil. Mag., 47, 209, 1924. [402] Payne, Proc. N. Ac. Sci., 11, 197, 1925. [Pg 116] CHAPTER VIII OBSERVATIONAL MATERIAL FOR THE TEST OF IONIZATION THEORY THE observational test of ionization theory involves a considerable program of measurement, if the accuracy necessary for a quantitative test is to be attained. The present chapter contains a synopsis of new data obtained by the writer to supplement the material already published in Harvard Circulars.[403][404] The data here presented practically complete the available material for the strong lines of known series relations in the region of the spectrum usually examined.