Carroll, Smith, Rao and Narayan and Schoepfle have established terms of the one-electron configurations of Pb IV. Schoepfle, whose analysis is the most complete, has identified the 6 s , 7 s , 8 s , 6 p 7 p , 6 d , 7 d , 5 f doublets, and in addition assigned a number of terms to the many-electron configurations that have as limit the 5 d 9 6 s configuration of Pb V. The Au I iso -electronic sequence, especially the configurations involving the 5 d 9 core, has been under investigation in this laboratory for some time. The analysis of Au I and of Hg II have been published and that of Pb IV is presented here. The investigation of Tl III is practically completed and will be published soon; Bi V is partially analysed. The state of our work on Tl III and Bi V is mentioned here because certain terms of these spectra and the corresponding terms of Pb IV have been established mainly by the regularity of the Moseley curves for these terms throughout the sequence Au I to Bi V. Thus the analysis of one spectrum has been facilitated and confirmed by a knowledge of the others. It is certain that some of the terms of Pb IV would not have been found except by such a method.

Three hundred and sixty lines of Bi IV, excited in an electrodeless discharge, have been classified. Twenty-seven terms of 6snx configurations, four terms of 6p 2 and six terms of 5d 9 6s 2 6p have been identified. Evidence ojf perturbation of 6s5/ and 6s7p by 5d 9 6s 2 6p and of 6s6d by 6p 2 is presented. The perturbation of 6s6d by 6p 2 in Bi IV and other members of the Hg I sequence is shown to agree qualitatively with Bacher's theory of sd-p 2 interactions. The hyperfine structure intervals of many of the terms of Bi IV have been evaluated. They are compared with the theoretical formulae for hyperfine structure separations and shown to be consistent with our multiplet analysis. Anomalous hyperfine structure has been observed in 6s7p±. This is interpreted as a coupling effect due to the approximate equality of the multiplet and hyperfine structure separations of 6s7p$. Goudsmit and Bacher's theory of hyperfine structure perturbations due to a change in coupling is applied to the four hyperfine levels of 6s7p\. Their theory accounts for the observed distribution of these levels and for the appearance of forbidden transitions.

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A single crystal of brucite, Mg(OH)2, containing manganese impurity to the extent of 100 parts per million, has been studied by means of the electron spin resonance (esr) technique. The study was made at room and liquid air temperatures. The observed absorption spectrum has been successfully interpreted in terms of parameters found in the appropriate spin-Hamiltonian. The measured values of these parameters at room temperature were found to be:[Formula: see text]

Fig. 2.-The opalescent spot at the surface is seen here as a bright cap at the top of the normally incident beam. The photograph is taken from above and the incidence also is from above.