RADIUM (Lat. radius, ray), was discov ered in 1898 at Paris by Prof. Pierre Curie and Madame Sklodowska Curie h collabora tion with Monsieur Bemont. It is the most important of over 30 newly discovered radio active elements, and was found in the sulphate residues from pitchblende of the Austrian mines at' Saint Joachinsthal, Bohemia, which had previously been worked for the •metal uranium. Radium is a chemical element, symbol Ra, atomic weight 226.0, atomic number 88, very similar to barium in its chemical properties. Radium accompanies barium in many chemical reactions and is separated from it by repeated fractional crystallization of the salts, usually the chloride Or bromide, the radium being en riched in the crystal fraction. A sufficient num ber of repetitions of the crystallization results in a pure radium salt, from which the atomic weight of radium and its spectral lines have been determined. The salts of radium most commonly prepared are the chloride and bromide (both readily soluble in water); the carbonate (soluble in adds) the sulphate (insoluble-100 times less soluble than barium sulphate). The salts are white when freshly prepared, have the same crystalline' form as the corresponding barium salts, but on standing rapidly change to yellow or brownish tints owing to the effect of their own radiations. The glass or silica tubes in which radium salts are usually retained also become colored by the radiations. The color is usually a bluish or deep violet, but some varieties of glass are colored brown. The color may be removed by raising the glass in temperature to a dull red glow. A most striking property of radium is its con tinual emission of radiations and heat. The heat emission from one gram of radium ele ment amounts to 132 small gram calories per hour. It was by means of its radiations that radium was originally discovered and by means of them that it can be detected in extreme dilu tion. For example, in' the ordinary commercial ores containing radium its content is about one part in • 150,000,000 by weight. None of the ordinary elements could be detected at this dilution and their concentration or recovery in a state of purity would be impotsible. Radio active elements emit three kinds of rays named for the first three letters of the Greek alphabet — alpha,• beta and gamma. The first two, be ing really corpuscular in nature, are more prop erly termed alpha and beta particles. Alpha particles have been proved to be helium atoms with 'a double Positive charge ot electricity, and are emitted at ver7 high velocities, one-fifteenth to one twentieth of the velocity of light. Beta
particles are electrons (single negative charge of electricity), and are emitted at various velocities from almost zero to that of light as a maximum. Gamma rays are ether vibrations (like 'ordinary light) but are of very short wave length and correspondingly high penetrat ing power. Those of the shortest wave length are 10 times more penetrating than the most powerful X-rays. The continuous emission of heat and radiant energy by radium at first greatly perplexed all scientists, as it appeared to controvert the law of the conservation of en ergy. The true explanation was proposed in 1903 by Rutherford (Sir Ernest E.) and Soddy (Prof. Frederick) on the hypothesis ot atomic disintegration or spontaneous transmutation of one element into another, accompanied by the emission of radiation. This theory has sub sequently been tested from all possible angles and has not only received the strongest experi mental confirmation, hut has initiated a new area of progress in the study of the constitution matter; and has incidentally placed the atomic theory on a solid basis of fact.
According to the theory of radioactive dis integration of the atoms, radium is only one member of the family of linear descendants from the parent element uranium (atomic weight 238). Each atomic change in the series is accompanied by the emission of a beta or an alpha particle, giving rise to an atom of the next lower member in the series, a new chem ical element wholly different in properties from the parent. These changes proceed with the utmost regularity according to a very definite law, namely, that the same fraction of change always takes place in the same interval of time (logarithmic equation). The interval in which 50 per cent of the element changes is called its half period, a very fundamental physical con stant, since the rate of change is independent of all physical or chemical conditions, such as temperature, pressure and concentration. The phenomenon is strictly a property of the atom. Therefore, all radium salts have the same radia tions as the radium metal itself. The metal was once prepared in a pure state by Mme Curie and was found to have all the properties to be expected of a base metal like barium. On account of the ease with which it oxidizes in air, it must he maintained in an atmosphere of hydrogen. The table on following page shows all the members of the uranium-radium family in their order of succession, their radiationi, 'half periods and atomic weight.