THE first Friday evening discourse of the new session at the Royal Institution was delivered on November 2 by Dr. F. W. Aston, who took as his subject “Elements and Isotopes”. That a chemical element could consist of isotopes of different atomic mass was first observed by Soddy when working on the products of radioactivity. Proof that this was true of the elements generally could only be obtained by direct atomic analysis. This was achieved by the mass-spectrograph, and with it the search for isotopes has been carried on continuously for the past fifteen years. Wide differences of properties among the elements necessitate very varied methods of obtaining the atomic rays required for the analysis. In some cases the technical difficulties are great; it is only during the last year that satisfactory results have been obtained with the rare earth group. Of the common elements, all but four, palladium, iridium, platinum and gold, have now been analysed and some 247 isotopes identified, a few by less direct optical methods. Elements of odd atomic num ber appear curiously limited to two isotopes, but elements of even atomic number can have many more, eleven in the case of tin. By means of modern instruments, it is possible to compare the masses of atoms to one part in ten thousand, an accuracy which it is expected to increase in the near future. These isotopic weights are required in order to test theories of nuclear structure, which have recently become of the greatest importance on account of the discovery of transmutation and of artificial produc tion of radioactive isotopes.