A mass spectrograph is described with which a study of the isotopic constitution of argon, potassium, rubidium, zinc and cadmium has been made. A calculation is carried out which indicates that the radioactivity of potassium and rubidium can best be explained by assuming that it is ${\mathrm{K}}^{40}$ in the case of potassium and ${\mathrm{Rb}}^{87}$ in the case of rubidium which is undergoing decay.

• Received 21 September 1936

DOI:https://doi.org/10.1103/PhysRev.50.1041