What is the nature of this energy?
The question does not appear to have been yet solved.
It certainly appears, according to Professors Haga and Wind and to Professor Sommerfeld, that with the X rays curious experiments of diffraction may be produced.

Dr Barkla has shown also that they can manifest true polarization.

The secondary rays emitted by a metallic surface when struck by X rays vary, in fact, in intensity when the position of the plane of incidence round the primary pencil is changed.

Various physicists have endeavoured to measure the speed of propagation, but it seems more and more probable that it is very nearly that of light.[27] [Footnote 27: See especially the experiments of Professor E.Marx (Vienna), _Annalen der Physik_, vol.xx.

(No.