By measuring the distribution of intensity of the diffuse radiation, Barkla was able to determine the total emission of a series of substances under varying conditions. One very important result among others that his investigations led to, enabled Barkla at an early stage to estimate approximately the number of electrons contained in an atom. The other variety of X-rays is wholly independent of the character of the incident radiation. Barkla showed that this radiation is homogeneous, that its absorption coefficient is not dependent on the incident radiation, but is determined by the irradiated substance. Further, he made the important discovery that the character of the rays is solely dependent on the qualities of the atoms constituting the substance, irrespective of their grouping and influence upon each other, that is to say, independent of the chemical composition of the substance.
Every chemical element yields a secondary radiation that is characteristic of that element.
Hence Barkla named this variety of radiation the characteristic X-radiation. This variety of radiation may be most conveniently studied in elements of relatively high atomic weight, for in them it is stronger than the diffuse radiation. The characteristic radiation, however, being, in contrast to the diffuse radiation, perfectly homogeneous, can be distinguished from the latter, and thus Barkla could trace the characteristic radiation down to elements of the atomic weight of 27. The characteristic radiation is furthermore, in contrast to the diffuse radiation, wholly unpolarized and is not dissymmetrical in respect to the direction of incidence of the primary radiation, but spreads uniformly in all directions. For this reason Barkla made a very thorough investigation of the absorption of X-rays by various substances.