In another subject, the nature of Light, many new and interesting speculations are being advanced as the result of the discoveries of the extraordinary properties of radium and the _x_-rays. To students of Theosophy these are significant, for H. P. Blavatsky, in _The Secret Doctrine_, goes deeply into the question whether light is an actual substance of some kind, or a mere undulation of an ethereal medium. She points out some of the difficulties of both theories, giving special attention to Sir W. Grove's celebrated lecture in 1842 wherein he considered he proved that light and heat must be affections of matter itself, and not the effects of an imponderable fluid--a finer state of matter--penetrating it. Sir Isaac Newton held to the Pythagorean theory that light was made of almost infinitely minute corpuscles, but the phenomenon of diffraction is supposed to have upset this. H. P. Blavatsky does not reject the wave theory as part of the explanation, but she contends that the ultimate causes of light, heat, and electricity must be sought in a form of matter existing in supersensuous states, states, though, "as fully objective to the spiritual eye of man as a horse or a tree to the ordinary mortal"; and, above all, that these forces and others are "propelled and guided by Intelligences." She devotes many chapters of the third part of the first volume of _The Secret Doctrine_ to this subject, throwing an entirely new light upon it in its deeper bearings, and showing the enormous importance of a proper understanding of it if we are ever to learn our true relationship with the external universe. She says: To know what light is, and whether it is an actual substance or a mere undulation of the "ethereal medium," Science has first to learn what are in reality Matter, Atom, Ether, Force. Now, the truth is, that _it knows nothing of any of these_, and admits it. (_The Secret Doctrine_, Vol. I p. 482) Since she wrote _The Secret Doctrine_, though hardly twenty-three years have elapsed, several discoveries in physics and chemistry have been made which have greatly modified the scientific view as to the nature of the atom, of the electric current, and of matter in general; all these modifications are leading straight in the direction of her teachings. It is even claimed that Matter can vanish without return.... Force and matter are two different forms of one and the same thing.... By the dissociation of matter, the stable form of energy termed matter is simply changed into those unstable forms known by the name of light, heat, etc. (_Evolution of Matter_, by Gustave Le Bon)