is a phenomenon which appears to depend upon the expulsion of certain electrons from substances, thus explaining the apparent conversion of one kind of matter into another as referred to under the heading of Atoms and this is done without materially disturbing the general character and properties of the residual substance in its relation to the groupings of the periodic law. In other words, elements may exist generally identical in chemical and physical properties, but having different atomic weights. Crookes originally found that when an electric current was passed through a glass tube previously exhausted of air to a great extent, certain rays looking like light, pass from the cathode to the anode, although the anode is the pole at which the current enters. These rays, called “radiant matter,” are able to drive a little vane placed in their paththat is, to exercise some small mechanical pressure. These emanations are now regarded as electrons, and will pass, as afterwards ascertained, through thin sheets of metal. The cathode rays may be made to converge by the use of an aluminium cup, thus producing a green phosphorescent spot on the glass. They travel in straight lines, and cast a strong shadow from any intervening object placed in their path; they also develop great heat, which may rise to the meltingpoint of platinum. The socalled X rays are a form of light lying beyond the visible end of the spectrum, and Eecquerel, in his search for the possible emission of Rontgen rays by fluorescent substances, encountered rays which are considered to be corpuscular. For example, a double salt of uranium and potassium, w ithout exposure to liht, was found to emit rays which affected a photographic plate, and this was the first ascertained instance of socalled radioactivity. This discovery was followed bj that of Curie and his wife, to the effect that the activity of the uranium compounds is due to the presence in them of some other very active substanceviz., radium, which proved to be a million times more active than uranium. Then it was found that radium itself emits three different types of radiation: one known as the alpha rays, which are unable to pass through a few sheets of paper; another, the beta rays which can be cut oft by a thick sheet of lead, and the gamma rays, of more intensely penetrating character. The alpha rays are regarded as atoms of helium, the beta rays as identical with Crookes

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Definition of  RADIO ACTIVITY