The nature of heat had long been discussed when the world of science decided in favor of the undulatory theory of light. One school held that the sensation of heat was caused by the cannonading of heat particles by the heated body; the other school, with few adherents, insisted that heat was simply a form of motion of the ether already adopted in the theory of light. The later discoveries of science proved with considerable certainty that the undulatory theory of heat is right, but it was well towards the middle of the last century before the emission theory of heat lost its ground. In fact, Dr.Whewell, in the third edition of his classic book on the _History of Inductive Sciences_, published in 1859, says that the undulatory theory of heat "has not by any means received full confirmation;"[A] and Dr. John Tyndall, in a book published in 1880, says, that the emission theory "held its ground until quite recently among the chemists of our own day."[B] Today, the evidences of modern science are overwhelmingly in favor of the undulatory theory of heat. [Footnote A: Vol. II, p. 184.] [Footnote B: Heat, A Mode of Motion, Tyndall, 6th ed., p. 38.] The wonderful developments of the last century, in electricity and magnetism, led to much speculation concerning the nature of the subtle electrical and magnetic forces. The most popular theories for many years were those that presupposed various electrical and magnetic fluids, which could be collected, conducted, dispersed and otherwise controlled. In 1867, the eminent English mathematician, Clerk Maxwell, proposed the theory that electrical and magnetic phenomena were simply peculiar motions of the ether, bearing definite relationship to light waves. Later researches, one result of which is the now famous Roentgen or X-rays, have tended to confirm Maxwell's theory. A recent text-book on physics, of unquestioned authority,[A] states that the ether theory of electricity and magnetism is now susceptible of direct demonstration; and another eminent authority frankly states that "when we explain the nature of electricity, we explain it by a motion of the luminiferous ether."[B] [Footnote A: Lehrbuch der Physik, Riecke, (1896), 2ter Band, p. 315.] [Footnote B: Popular Lectures and Addresses, Kelvin (1891) Vol. 1, page 334.] Other recent discoveries have hinted at the possibility of matter itself being only the result of peculiar forms of this all-pervading substance, the luminiferous ether. The properties of the element radium, and other radioactive elements, as at present understood, suggest the possibility of a better understanding of the nature of the ether, and of its relation to the world of phenomena. [Sidenote: The existence of the ether is a certainty of science.]