mesothorium i

Mesothorium \Mes`o*tho"ri*um\, n. [NL.; meso- + thorium.]
   (Chem.)
   a radioactive isotope of radium (radium-228) with a half-life
   of 5.8 years. Also called {mesothorium-1} or {mesothorium I}
   to distinguish it from a subsequent decay product,
   mesothorium II (actinium-228). It was discovered in 1907 by
   Otto Hahn as a decay product of thorium (produced by decay of
   thorium-232). Mesothorium-1 (radium-228) in turn produces
   actinium-228 (mesothorium-2) as the first product of its
   radioactive decay, and the actinium-228 in turn decays
   quickly (half-life of 6 hours) to thorium-228 (which is also
   called {radiothorium}; the thorium-228 has a half-life of
   1.91 years, shorter than that of the radium-228). It was
   discovered and named before full recognition of the nature of
   isotopes of the elements, and was distinguished from other
   variants of radium by its half-life and mode of production
   and decay. It was also cheaper to prepare than other
   short-lived radium isotopes, and was thus sold commercially,
   for use, e.g. in making watch dials readable in the dark by
   painting the hands and hour marks with a self-luminous paint
   containing the radioactive substance; it is therefore often
   referred to (e.g. in regulatory legislation) as though
   distinct from radium. It was one of the isotopes believed
   responsible for radiation-induced diseases observed in
   industrial workers who painted radium on watch dials in the
   late 1950's and early 1960's. The primary isotope of radium
   (radium-226) has a half-life of 1620 years, and these
   isotopes with shorter half-lives proved difficult to isolate
   and study for the purpose of finding the cause of such
   diseases.
   [Webster 1913 Suppl. +PJC]