A considerable amount of work has been done by various experimenters showing that, when an element of higher atomic weight than calcium is subjected to a suitable primary beam of X-rays, the rays which leave the radiator consist of two types: firstly, the purely scattered radiation, which is almost exactly similar to the incident beam, and, secondly, a characteristic homogeneous radiation. The scattered radiation which in the case of a primary beam from an X-ray bulb is heterogeneous, is, with elements of low atomic weight, quite small in intensity when compared with the intensity of the homogeneous radiation which is emitted simultaneously. Owing to this fact, it is comparatively easy to prove that the elements with atomic weights between that of calcium and cerium give off when stimulated with X-rays homogeneous beams, and the hardness of the characteristic radiation from each of these elements has been measured by determining the absorption in aluminium. The radiations are usually defined by the value ok their absorption coefficients, that is, by λ/ρ where I = I
0
e
-λx
; ρ = density of aluminium. Using the values obtained, it is possible to plot a curve showing the relation between atomic weight and λ/ρ for the elements which emit a characteristic radiation, taking atomic weight as abscissa and λ/ρ for ordinates. If this is done, it will be found that the elements with atomic weights between that of calcium and cerium lie on an approximately smooth curve (Group K). When, however, the elements with higher atomic weight than silver are examined under suitable conditions, it is found that, with these elements, there are two distinct types of radiations: one, a hard characteristic radiations such as belongs to Group K, and superposed on this a very soft radiation. Prof. Barkla and Mr. Nicol have investigated the soft radiations from the elements silver, antimony, iodine, and barium, and have shown that these elements, in addition to the usual characteristic radiation, emit another very soft radiation, which is also characteristic of the element. The values of the λ/ρ for these elements have been determined, and it has been shown, as far as it is possible with such soft rays, that they are homogeneous. If these values are plotted on the same diagram as that mentioned above, a second short curve is obtained, which can be continued to the X axis; when this is done, if this second curve resembles in shape the curve for Group K, it will pass before it reaches the X axis through the region of atomic weights between 184 and 238. which contains tubgsteb, gold, platinum, lead, bismuth, thorium, and uranium. This second series of elements has been designated Group L. Up to the present it has been impossible to draw this curve with any accuracy, as none of the elements between tungsten and uranium have been investigated as regards their X-ray properties.
Observation of electron-induced characteristic x-ray and bremsstrahlung radiation from a waveguide cavity