Source : Webster's Revised Unabridged Dictionary (1913)

Radium \Ra`di*um\, n. [NL., fr. L. radius ray.] (Chem.)
   An intensely radioactive metallic element found (combined) in
   minute quantities in pitchblende, and various other uranium
   minerals. Symbol, Ra; atomic weight, 226.4. Radium was
   discovered by M. and Mme. Curie, of Paris, who in 1902
   separated compounds of it by a tedious process from
   pitchblende. Its compounds color flames carmine and give a
   characteristic spectrum. It resembles barium chemically.
   Radium preparations are remarkable for maintaining themselves
   at a higher temperature than their surroundings, and for
   their radiations, which are of three kinds: alpha rays, beta
   rays, and gamma rays (see these terms). By reason of these
   rays they ionize gases, affect photographic plates, cause
   sores on the skin, and produce many other striking effects.
   Their degree of activity depends on the proportion of radium
   present, but not on its state of chemical combination or on
   external conditions.The radioactivity of radium is therefore
   an atomic property, and is explained as result from a
   disintegration of the atom. This breaking up occurs in at
   least seven stages; the successive main products have been
   studied and are called

   {radium emanation} or exradio,

   {radium A},

   {radium B},

   {radium C}, etc. (The emanation is a heavy gas, the later
      products are solids.) These products are regarded as
      unstable elements, each with an atomic weight a little
      lower than its predecessor. It is possible that lead is
      the stable end product. At the same time the light gas
      helium is formed; it probably consists of the expelled
      alpha particles. The heat effect mentioned above is
      ascribed to the impacts of these particles. Radium, in
      turn, is believed to be formed indirectly by an
      immeasurably slow disintegration of uranium.