In addition to the critical potentials, which give a measure of the ease with which an atom is excited or ionized, astrophysical theory requires an estimate of the readiness with which an atom recovers after excitation or ionization. It appears probable that this factor, like the critical potentials, is independent of external conditions, and depends upon something that is intrinsic in the atomic structure. The “life” of the atom has been extensively investigated in the laboratory, and has been shown to be a small fraction of a second in duration. Probably this subject of “atomic lives” is still in an initial stage, and the accuracy of the results and the range of elements discussed will be greatly increased in the near future. A summary of the material obtained up to the present time is contained in the following table. Successive columns contain the atom discussed, the deduced atomic life in seconds, the authority, and the reference. The data are practically confined to hydrogen and mercury, and for both these elements the atomic life appears to be of the order [Pg 22] . Astrophysical estimates of the life of the excited calcium atom have been made by Milne,[20] who derives values of the order . This is so near to the values obtained in the laboratory that it seems permissible, in the absence of further precise data, to assume an atomic life of , as a working hypothesis, for all atoms. The same value is unlikely to obtain for all atoms; in particular it may be expected to differ for atoms in different states of ionization. But here astrophysics must be entirely dependent on further laboratory work for the determination of a quantity that is of fundamental importance. TABLE IV Atom Life Authority Reference Wien An. d. Phys., 60, 597, 1919 Ibid. Ibid. Ibid. Ibid. Ibid. Ibid. Dempster Phys. Rev., 15, 138, 1920 Wood Proc. Roy. Soc., 99A, 362, 1921 g Franck and Grotian Zeit. f. Phys., 4, 89, 1921 , Mie An. d. Phys., 66, 237, 1921 , Wien An. d. Phys., 66, 232, 1921 bands Ibid. Ibid. , Ibid. An. d. Phys., 73, 483, 1924 Ibid. Ibid. Ibid. Ibid. Ibid. Ibid. Turner Phys. Rev., 23, 464, 1924 Webb Phys. Rev., 21, 464, 1923 RELATIVE PROBABILITIES OF ATOMIC STATES