=421. The Electromagnetic Theory of Light.=--The study of electric waves has shown that they are similar to light waves in many respects: (a) they have the same velocity; (b) they can be reflected and refracted. The main difference is in their length, light waves being very much shorter. In 1864 James Clerk Maxwell, an English physicist, proposed the theory that ether waves could be produced by electrical means and that light waves are electromagnetic. In 1888 Hertz proved by his experiments that ether waves having the same velocity as light could be produced in this way. It is now the general belief that light waves are ether waves produced by the vibrations of the electrons within the atoms and that they consist of electromagnetic waves in the ether. =422. Radio-activity.=--In 1896 Henri Becquerel of Paris discovered that uranium and its compounds emit a form of radiation that produces an effect upon a photographic plate that is similar to that resulting from the action of "X" rays. These rays are often called _Becquerel_ rays in honor of their discoverer. The property of emitting such rays is called =radio-activity=, and the substances producing them are called =radio-active=. In 1898, Professor and Mme. Curie after an investigation of all the elements found that _thorium_, one of the chief constituents of incandescent gas mantles, together with its compounds, was also radio-active. This may be shown by the following experiment: Place a flattened gas mantle upon a photographic plate and leave in a light tight-box for several days. Upon developing the plate in the usual way a distinct image of the mantle will be found upon the plate. =423. Radium.=--Mme. Curie discovered also that pitch-blende possessed much greater radio-active power than either thorium or uranium. After prolonged chemical experiments she obtained from several tons of the ore a few milligrams of a substance more than a million times as active as thorium or uranium. She called this new substance _radium_. Radium is continually being decomposed, this decomposition being accompanied by the production of a great deal of heat. It has been calculated that it will take about 300 years for a particle of radium to be entirely decomposed and separated into other substances. It is also believed that radium itself is the product of the decomposition of uranium, atomic weight 238, and that the final product of successive decompositions may be some inert metal, like lead, atomic weight 207. The radiation given off by radio-active substances consists of three kinds: (A) Positively charged particles of helium called _alpha_ rays: (B) negatively charged particles called _beta_ rays: (C) _gamma_ rays.