Positive rays are sorted into an extremely thin ribbon by means of parallel slits S1S2, and are then spread into an electric spectrum by means of the charged plates P,P2. A portion of this spectrum deflected through an angle @ is selected by the diaphragm D and passed between the circular poles of a powerful electromagnet O the field of which is such as to bend the rays back again through an angle & more than twice as great as 0. The result of this is that rays having a constant mass (or more correctly constant m/e) will converge to a focus F, and that if a Fig. 1. Diagram of positive-ray spectrograph. photographic plate is placed at GF as indicated, a spectrum dependent on mass alone will be obtained. On account of its analogy to optical apparatus, the instrument has been called a positive-ray spectrograph and the spectrum produced a massspectrum. = Plate IV shows a number of typical mass-spectra obtained by this means. The number above the lines indicates the masses they correspond to on the scale O 16. It will be noticed that the displacement to the right with increasing mass is roughly linear. The measurements of mass made are not absolute, but relative to lines the mass of which is known. Such lines, due to hydrogen, carbon, oxygen, and their compounds, are generally present as impurities or purposely added, for pure gases are not suitable for the smooth working of the discharge tube. The two principal groups of these reference lines are the C, group due to C (12), CH (13), CH2 (14), CH, (15), CH, or O (16), and the C2 group 24-30 containing the very strong line 28, C2H1 or CO. In spectrum i. the presence of neon is indicated by the lines 20 and 22 situated between these groups. Comparative measurements show that these lines are 20.00, 22.00 with an accuracy of one-tenth per cent., which removes the last doubt as to the isotopic nature of neon. The next element investigated was chlorine; this is characterised by four strong lines 35, 36, 37, 38, and fainter ones at 39, 40; there is no trace of a line at 35-46, the accepted atomic weight. From reasoning which cannot be given here in detail it seems certain that chlorine is a complex element, and consists of isotopes of atomic weights 35 and 37, with possibly another at 39. The lines at 36, 38 are due to the corresponding HCl's. Particles with two, three, or more electronic charges will appear as though having half, a third, etc., their real mass. The corresponding lines are called lines of the second, third, or higher order. In spectrum ii. the lines of doubly charged chlorine atoms appear at 17.5 and 18.5. Analyses of argon indicate that this element consists almost entirely of atoms of weight 40, but a faint component 36 is also visible. Spectra v. and vi. are taken with this gas present; the former shows the interesting third order line at 131. Krypton and xenon give surprisingly complex results; the former is found to consist of no fewer than six isotopes, the latter of five (spectra viii. and ix.). Mercury is certainly a complex element probably composed of five or six isotopes, two of which have atomic weights 202 and 204; its multiply charged atoms give the imperfectly resolved groups, which are indicated in several of the spectra reproduced in Fig. 2. By far the most important result obtained from this work is the generalisation that, with the exception of hydrogen, all the atomic weights of all elements so far measured are exactly whole numbers on the scale O 16 to the accuracy of experiment (1 in 1000). By means of a special method (see Phil. Mag. May, 1920, p. 621), some results of which are given in spectrum vii., hydrogen is found to be 1.008, which agrees with the value = accepted by chemists. This exception from the whole number rule is not unexpected, as on the Rutherford "nucleus" theory the hydrogen atom is the only one not containing any negative electricity in its nucleus. The results which have so far been obtained with eighteen elements make it highly probable that the higher the atomic weight of an element, the more complex it is likely to be, and that there are more complex elements than simple. It must be noticed that, though the whole number rule asserts that a pure element must have a whole number atomic weight, there is no reason to suppose that all elements having atomic weights closely approximating to integers are therefore pure. The very large number of different molecules possible when mixed elements combine to form compounds would appear to make their theoretical chemistry almost hopelessly complicated, but if, as seems likely, the separation of isotopes on any reasonable scale is to all intents impossible, their practical chemistry will not be affected, while the whole number rule introduces a very desirable simplification into the theoretical aspects of mass. THE NATURE OF X-RAYS AND THE THE phenomena of light can only be explained by supposing that light is a wave motion. If a parallel beam of white light be allowed to fall on a grating, that is, a transparent or reflecting surface ruled with fine scratches of many thousands to the inch, the transmitted or reflected light will show the colours of the rainbow. From the angular deflexion of any particular colour its wave length can be measured in comparison with the distance between the scratches. Röntgen rays did not show these diffraction spectra with ordinary gratings, and for some time their nature was doubtful. But it was suggested by Laue that the regular arrangement of atoms or molecules in a crystal might serve as a grating of very minute dimensions. X-rays have been examined in this way, especially by Sir Wm. Bragg and his son W. L. Bragg, and shown to be similar to waves of light of very short wave-length. The structure of crystals has also been elucidated. The diffraction patterns of X-rays can be photographed, and hence their spectra examined. The spectra are found to depend on the nature of the anti-cathode which serves as the source of the rays, and, by using different elements as anti-cathode, their characteristic X-ray spectra can be mapped, the same crystal being employed as grating throughout. A very interesting and important work on these lines was done by G. H. J. Moseley, of Eton and Oxford, who carried on his research in Sir E. Rutherford's laboratory at Manchester, joined the army in 1914 and lost in the war a life of the greatest promise to his country and the world. Moseley discovered that the square roots of the frequency of vibration of the chief lines in the X-ray spectra increased regularly as he passed from element to element up the Periodic Table. He thus assigned to each element an atomic number, giving its true place in the Periodic Table, and the number of unit positive charges in the nucleus of its atom. This number is found to be fundamental in the modern theory of atomic structure and on it depend the chemical properties of the element. The HIGH FREQUENCY SPECTRA OF THE ELEMENTS By H. G. J. Moseley, m.a. Phil. Mag. (1913), p. 1024. In the absence of any available method of spectrum analysis, the characteristic types of X radiation, which an atom emits if suitably excited, have hitherto been described in terms of their absorption in aluminium. The interference phenomena exhibited by X-rays when scattered by a crystal have now, however, made possible the accurate determination of the frequencies of the various types of radiation. This was shown by W. H. and W. L. Bragg, who by this method analysed the line spectrum emitted by the platinum target of an X-ray tube. C. G. Darwin and the author extended this analysis and also examined the continuous spectrum, which in this case constitutes the greater part of the radiation. Recently Prof. Bragg has also determined the wave-lengths of the strongest lines in the spectra of nickel, tungsten, and rhodium. The electrical methods which have hitherto been employed are, however, only successful where a constant source of radiation is available. The present paper contains a description of a method of photographing these spectra, which makes the analysis of the X-rays as simple as any other branch of spectroscopy. The author intends first to make a general survey of the principal types of high-frequency radiation, and then to examine the spectra of a few elements in greater detail and with greater accuracy. The results already obtained show that such data have an important bearing on the question of the internal structure of the atom, and strongly support the views of Rutherford and of Bohr. ·R' R Fig. 1. Kaye has shown that an element excited by a stream of sufficiently fast cathode rays emits its characteristic X radiation He used as targets a number of substances mounted on a truck inside an exhausted tube. A magnetic device enabled each target to be brought in turn into the line of fire. This apparatus was modified to suit the present work. The cathode stream was concentrated on to a small area of the target, and a platinum plate furnished with a fine vertical slit placed immediately in front of the part bombarded. The tube was exhausted by a Gaede mercury pump, charcoal in liquid air being also sometimes used to remove water vapour. The X-rays, after passing through the slit marked S in Fig. 1, emerged through an aluminium |