and profound meditation he was convinced of its truth, and gave his reasons for his changed opinions in an essay which received the prize and was read to the academy in 1819, but which for some reason was not published till 1826. The difficulty in explaining transverse vibrations was overcome by showing that polarized rays of light have laterality, and by showing how a stretched string vibrates. The mechanical difference between light and sound was discovered. The tremor which causes sound, it was shown, is in the elastic fluid which carries the movement forward. Without motion there can be neither vision nor hearing. As motion is required to explain the behavior of gases and that fact had been set forth clearly in the experiments of Clausius, Maxwell and Joule, the conclusion did not seem to be far fetched that motion exists everywhere, and must be considered in whatever explanation one attempts to make of physical phenomena. It was also discovered that bodies moving rapidly round an axis, when immersed in water or in any movable medium, cease their rotation and flow forward as other bodies flow in water. In 1857 Helmholtz and Lord Kelvin brought forward the vortex theory to explain cosmic conditions and to account for the formation of the planetary worlds. The studies and experiments of Faraday in electricity and magnetism seemed to favor this new theory of motion. It was shown that the medium which carries light and sound can not possess the ordinary properties of a solid, a liquid or a gas. Hence a demand for a medium in which everything may exist, through the aid of which all movements or vibrations may be conveyed. Careful studies discovered the velocity which particles of hydrogen, for example, obtain, and Faraday pointed out lines of force, afterwards called "tubes of force" by Hertz, along which in a magnetic field, the electric movement passes. These movements were measured mathematically and with such care and accuracy that not only is the course which electricity takes known but its velocity also. But even if electricity be displacement, and its track be accurately discovered and followed, no one pretends to know just what is its innermost nature or denies that future study may entirely change present opinions as to its nature and its laws.
It has been proved, scientists declare, that all forces of matter may be measured and expressed in terms of energy or motion. During the last twenty years the conception has been formed that light is an electric or a magnetic phenomenon. Luminous waves are short rapidly moving waves and difference in color is caused by difference in length and frequency. But what is most important in this theory of motion is that it is not necessary, as in the gravitation theory or the atomic theory, to believe in the theory of action at a distance. Bodies move because other bodies press upon them. We see and hear because some sort of motion brings the ether into connection with eye and ear. The fact of gravity is not denied. Nor is it denied that Newton's dis-
