X rays had been discovered eleven years earlier and were still of profound interest to physicists. The British physicist Charles Glover Barkla (1877-1944) studied the manner in which X rays were scattered by gases and found that the higher the molecular weight of the gas, the greater the scattering of the X rays. From this he deduced, in 1904, that the more massive the atoms and molecules, the more charged particles they contained, since it was the charged particles that did the scattering. This was the first indication of a connection between the number of charged particles in an atom and its position in the periodic table.
He further showed, from the manner of scattering, that X rays were transverse waves like light, not longitudinal waves like sound. This was the final proof that they were examples of electromagnetic radiation.
In 1906 Barkla went on to something still more important. He showed that when X rays were scattered by particular elements, they produced a beam with a particular degree of penetration. The higher the atomic weight of an element, the more penetrating the characteristic X rays they produced. He went on to describe two types of such X rays: the more penetrating he called K radiation and the less penetrating he called L radiation.
At the time it was difficult to know what to make of this, but before long, characteristic X rays were to be important in rationalizing the periodic table.
For his work on X rays, Barkla was awarded the Nobel Prize in physics in 1917.