The absorption spectrum given by the reversing layer when it is at a low temperature will consist of the lines given most readily by the atom in its normal state. The energy transfers which move an electron from its normal orbit to another correspond to the “ultimate lines,” and these lines will therefore be especially outstanding in the spectra of the coolest atmospheres. They are of such importance, from theoretical and from practical standpoints, that a list of them is reproduced here. Successive columns of the table give the atomic number and atom, the ionization potential, the wave-lengths of the ultimate lines, and an indication of their observed occurrence in stellar spectra. An asterisk denotes that the line has been observed, and a dash indicates that it has not been recorded. [Pg 95] It may be remarked that the ultimate lines of sodium, potassium, lithium, rubidium, and caesium are in the visible region—a fact which is utilized in the laboratory flame tests used in qualitative analysis.[363] The brilliancy of the flame colors obtained in the Bunsen burner, at the temperature of about 1500°C., is a striking elementary illustration of the readiness with which the atom in its normal state will take up and re-emit the frequency corresponding to the ultimate lines (second pair for K, Rb, Cs). TABLE XVII Atom Ionization Potential Wave-lenght Stellar Ocurrence 1 H 13.54 1215 - 2 He 24.47 584, 557 - 3 Li 5.37 6707 * 4 Be ? 2349 - 5 B ? 2498, 2497 - 6 C ? 2479 - 7 N ? - - 8 O 13.56 1306, 1304, 1302 - 9 F ? - - 10 Ne 16.7 - - 11 Na 5.12 5896, 5890 * 12 Mg 7.61 2852 - 13 Al 5.96 3962, 3944 - 14 Si ? 2882 - 15 P ? 2553, 2536 - 16 S 10.31 1915, 1900 - 17 Cl ? - - 18 A ? - - 19 K 4.32 7699, 7665 * 20 Ca 6.09 4227 * 21 Sc ? 4247, 3652 * 22 Ti 6.5 5065, 5040, 5014 * 23 V ? 4331, 4333 * 24 Cr 6.75 4290, 4275, 5254 * 25 Mn 7.41 4034, 4033, 4031 * 26 Fe ? 2756, 2749 - 27 Co ? 3454, 3405 * 28 Ni ? 3415 * 29 Cu 7.69 3274, 3248 * [Pg 96] 30 Zn 9.35 2139 - 31 Ga 5.97 4172, 4033 * 37 Rb 4.16 7948, 7800 * 38 Sr 5.67 4607 * 40 Zr ? 4496, 4392 * 42 Mo ? 3903, 3864, 3798 * 47 Ag 7.54 3383, 3281 * 48 Cd 8.95 2288 - 49 In 5.76 4511, 4102 * 50 Sn ? 3262 ? 55 Cs 3.88 8943, 8581 - 56 Ba 5.19 7911 - 57 La ? 3949 * 79 Au 8.72 2676, 2428 - 80 Hg 10.39 2537, 1850 - 81 Tl 6.08 5350, 3775 - 82 Pb 7.38 4058, 3684 *