The bonds shall not bear a greater amount of interest than 6 percent, to be payable annually or semiannually. The bonds shall be sold in the manner prescribed by the board of supervisors, but for not less than par, and the proceeds of the sale thereof shall be deposited in the county treasury to the credit of the building fund of the library district, and shall be drawn out for the purposes for which the bonds were issued as other library money is drawn out.