All money levied, when collected, shall be paid into the county treasury to the credit of the library district, and shall be used for the payment of principal and interest on the bonds, and for no other purpose. The principal and interest on the bonds shall be paid by the county treasurer, upon the warrant of the county auditor, out of the fund provided therefor. The county auditor shall cancel and file with the county treasurer the bonds and coupons as rapidly as they are paid.