Definition of 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
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Origin and Etymology of mesothorium
New Latin, from mes- + thorium
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