Henry Stuart , Duke of Gloucester ( 8 July 1640 â `` 18 September 1660 ) was the third adult ( and youngest ) son of Charles I and his queen , Henrietta Maria of France . Henry Stuart , 1st Duke of Gloucester ( 8 July 1640 â `` 18 September 1660 ) was the third adult ( and youngest ) son of Charles I and his queen , Henrietta Maria of France .