Not less than fifteen percent of the tax imposed for severing coal shall be placed into a permanent trust fund in the state treasury to be held in trust and administered by the board of university and school lands, which shall have full authority to invest said trust funds as provided by law, and may loan moneys from the fund to political subdivisions as provided by law. The interest earned on the moneys in said trust fund shall be used first to replace uncollectable loans made from the fund, and the balance shall be credited to the general fund of the state.