show identical general physical and chemical behavior. For example, the elements radium B, radium D and lead, of atomic weights 214, 210, and 207, respectively, are so closely allied in chemical and physical properties that all attempts to separate a mixture of any two of them have failed completely. This would be explained if the nuclear charges were identical for those elements, as the generalization, already referred to, indicates. If this be the case, they should give identical spectra under similar conditions. Unfortunately the elements radium B and radium D are in too small quantity to determine their ordinary light spectra, but we can compare the X-ray spectrum of lead with that given by radium B under the excitation of its own beta rays. Experiments of this kind were recently made by Dr. Andrade and the writer, and the two spectra were found to be identical within the limits of experimental error. It is to be anticipated that their light spectra would also prove to be identical, or nearly so, for, as previously pointed out, the effect of the mass of the nucleus on the spectrum is probably very small.
The fact that the atoms of these three elements are not identical as regards mass or radioactive properties, shows that the structure of the nucleus is different in each case.
There is another important deduction that should be mentioned. The end product of the uranium-radium series is an inactive element which has long been considered to be lead, but it has been difficult to verify this conclusion by direct experiment. We have seen that the end product has the same atomic number as lead, but should have an atomic weight about 206 instead of 207 as found for ordinary lead. In a similar way, it has been concluded by Soddy and Fajans that the end product of thorium has the same atomic number as lead, but should have an atomic weight about 208.5. In order to test these remarkable conclusions, experiments are now in progress by a number of investigators in different countries to examine whether the lead always found in radioactive minerals and which presumably has partly, if not wholly, a radioactive origin, shows the same atomic weight as ordinary lead. Soddy has already found evidence that there is a distinct difference in the atomic weights in the direction predicted by the theory[1].
The question naturally arises whether some of the ordinary elements may not prove to be a mixture of two, or even more, of these "isotopes," as they have been termed. Unless the component isotopes are present in different proportion in different natural sources of the element, it will be difficult to settle this problem by ordinary chemical methods.
- ↑ Since the delivery of this lecture, similar conclusions have been reached by the experiments of Richards in Cambridge and Hönigschmid in Vienna. There still, however, remains some doubt as to the actual difference in atomic weight of uranium lead, thorium lead and ordinary lead. A very promising beginning has thus been made on the attack of this most important and fundamental problem.
