In continuation of the previous communications on the isotopic constitution of the elements determined by photometry of their mass-spectra this paper contains an account of experiments made on lead. Much of the work was done over a year ago and, on account of their great importance in geological problems, some of the numerical results were published. It seemed desirable that the complete account should include, if possible, results of at least one of those abnormal samples reported to have an atomic weight less than 206. These have now been obtained and as the mass-spectrograph is now being partially reconstructed and may not be available for further work of the same kind for some time further delay is unnecessary. Lead is unique among the elements in that it is formed in workable amounts by different processes of radioactive disintegration and so does not have a constant isotopic constitution. The first mass-spectra showing its lines were obtained with the second mass-spectrograph by a discharge in the tetramethyl made from ordinary lead and indicated isotopes 206, 207, 208 roughly in proportions 4, 3, 7 with possibly others. Two years later a specimen of the methyl compound of lead from Norwegian Broggerite was prepared and analysed. The mass-spectra obtained were poor but enabled the three lines 206, 207, 208 to be identified and their relative intensities roughly estimated at 100, 10·7, and 4·5. At the time there was no reason to suppose that the intensities could be affected by the presence of a hydride, so the percentages were worked out at 86·8, 9·3, 3·9. The presence of line 207 was of the greatest interest for, as was pointed out, there was excellent reason for identifying this with the final product of the actinium series, which would settle the atomic weights of the members of the series, that of protactinium being 231. Assuming the ratio 100 : 7 for the number of atoms of uranium lead to actinium lead Lord Rutherford was able to work out the period of actino-uranium, the parent of the actinium series and an isotope of uranium, to be 4·2 x 10
8
years, and deduce other important cosmical conclusions.
The isotopic constitution and atomic weight of lead from different sources