mesothorium

:  either of two radioactive products intermediate between thorium and radiothorium in the thorium series or a mixture of the two products obtained usually from thorium minerals (as monazite sand) and used as a substitute for radium especially in luminous paints: a :  an isotope of radium —called also mesothorium 1 —symbol MsTh₁ or Ra²²⁸ b :  an isotope of actinium —called also mesothorium 2 —symbol MsTh₂ or Ac²²⁸