The proceeds of any such bonds shall not be applied to, or used for, any purpose or purposes, other than that for which such bonds are issued, and such proceeds shall, until properly disbursed, be kept in a fund separate and apart from all proceeds of other bond issues and other funds of the issuing corporation; provided, however, that when, in the judgment of the governing board of the issuing corporation, the proceeds of any bond issue should be temporarily invested pending application of such proceeds to the purposes for which the bonds were issued, the governing board of the issuing corporation may invest the proceeds of such issue or any part thereof in registered securities of the United States. Any unexpended balance remaining after the accomplishment of such purpose or purposes shall be used and applied only to and for the payment and redemption of such bonds and the payment of interest thereon, pro tanto.