The treasurer shall loan at the best legal rate of interest he can procure, on the pledge of the United States stock, at a rate not above its par value, as collateral security, any surplus money in the treasury not otherwise appropriated nor already authorized to be invested by the trustees of the school fund as the same shall from time to time accrue. He shall loan the same on such terms of credit as will enable him to receive such moneys again, in order to meet the demands of the treasury.