When such bonds shall have been duly signed and countersigned and registered, the same shall be deposited with the county treasurer of such county, and by such county treasurer negotiated and sold for not less than par value; and the proceeds arising from the sale of said bonds shall be deposited in the county treasury to the credit of said irrigation district, and shall be devoted to purchasing or building a main ditch, ditches and laterals, head gates, dams and all other works, or any thereof, for purposes of irrigation as herein provided.