Most of the X-ray spectra consists of a very easily absorbed radiation so that in using an ordinary X-ray tube they will not reach outside the glass-walls. In the diagram in fig. 9, only the small portion between the vertical lines o and r A.U. in cludes the radiation given off by the technical X-ray tube. Hence for the study of X-ray spectra, one is confined to the use of tubes with thin foils as windows and spectrometers built for vacuum.
The study of these spectra has shown that there is always one absorption-edge in the region of wave-lengths where the K-group of the same element is located. In the region of the L-group there are three absorption-edges and finally in the domain of the M-group experiments have revealed five edges.
As to the K-absorption-edges the measurements of their wave lengths show that they have, if not exactly, very nearly, the same values as those of the line with the shortest wave-lengths within the K-group.
How the three absorption-edges of the L-group are located relative to the L-emission-lines is shown in figure ro. As seen from this diagram, here also the edge with the shortest wave length agrees with the shortest emission-line. It is reasonable to suppose that the L-group may be divided in three sub-groups of lines of which every one is related to one of the three edges as is the case in the K-group. A numerical analysis of the material strongly supports this view as will be stated later.
Analogous results concerning the five edges within the M group indicate the existence of five sub-groups in this series.
Connection Between Emission- and Absorption-spectra. —As soon as reliable measurements of the two kinds of X-ray spectra, line- and absorption-spectra, had been performed it was immediately clear that there existed a very intimate relation be tween them. If the frequencies of the absorption-edges are indi cated by K for the one absorption-edge within the K-group L. for the three absorption-edges within the L-group M. My for the five absorption-edges within the M-group it is found that the differences between some of these values give exactly the frequencies of some of the emission-lines. For in
stance the differences and K —Lin give the values of the frequencies of the two strongest lines of the K-group namely and On calculating the difference between some of the above M-values and the K-value other lines belonging to the K group are obtained.
In the same way a difference between and lam with some of the values of the higher absorption-edges (MI • • •) gives emission-lines of the L-group. Now these connections between the absorption and emission-spectra allow an extension which seems highly probable. The frequencies obtained by calculating the difference between the three L-values and the M-values include only part of the L-emission-lines, but supposing the ex istence of higher absorption-frequencies Nm • • • Or • • • of suitable frequencies the remaining L-lines may be accounted for as differences between absorption-frequencies. The existence of these higher absorption edges has not as yet been directly experimentally proved but they are strongly supported by the fact that also lines of the M-group may be calculated from them in the same way as were the L-lines.
If this hypothesis is right we should expect that there are for the heaviest elements seven absorption-edges, five 0 and prob ably three P-edges.
Now it may be added that not all the numerical differences between two arbitrary absorption-values correspond to emission lines. Some of these "calculated" lines for some reason or other do not exist ; they belong to what are called "forbidden" transitions. Physical Interpretation of the X-ray Line-spectra and the Structure of the Atom.—According to the Bohr-Ruther ford hypothesis the atoms are built up of a positive nucleus of comparatively small dimensions which is surrounded by a number of electrons. If this general picture of the atom is accepted the X-ray spectra furnishes us with a lot of valuable information regarding the feature of the electronic part of the atom. Then plausible calculations show that the X-ray series of highest fre quencies, the K-series, must be ascribed to those electrons which form the innermost part of the electronic atmosphere of the atom. The higher series consequently correspond to successive electronic transitions outwards to the surface-layers of the atom, where we meet the region of the ordinary optical spectra.