Any funds derived from the voted special school taxes levied in certain cities and school districts in Calhoun county under the provisions of that certain amendment to the Constitution proposed by Act No. 587 enacted at the 1947 regular session of the legislature of Alabama [Amendment No. 68] that are not needed to pay debt service on bonds issued under said amendment, or to comply with any other covenants contained in proceedings authorizing the issuance of such bonds, may, to such extent as the governing body of the political subdivision or agency entitled thereto may determine are excess of the amount then needed for the specific public school purpose for which said taxes were voted, be used for public school purposes generally. Any bonds which are now outstanding or which may hereafter be issued under the provisions of said amendment or this amendment may at any time and from time to time be refunded, whether before, at or after the maturity of the bonds refunded, by the issuance of new bonds, payable from the same sources as those refunded, in a principal amount sufficient to pay said bonds so refunded and any premium necessary to redeem, pay, purchase or otherwise retire said bonds; and bonds may be issued for the combined purpose of so refunding any such bonds and obtaining funds for acquiring, constructing and improving public school buildings, including sites therefor. Notwithstanding the fact that they may be payable solely from a specified source, all bonds hereafter issued under the provisions of said amendment or this amendment shall be negotiable instruments within the meaning of the negotiable instruments law of Alabama if they otherwise possess all the characteristics of negotiable instruments under the laws of Alabama and shall be legal investments for trust funds. Bonds authorized to be issued hereunder shall be issued in the same manner and by the same respective political subdivisions as those authorized to be issued under said amendment proposed by said Act No. 587 [Amendment No. 68] and may be so issued without the necessity of any further election. No bonds issued hereunder shall be chargeable against the constitutional debt limit of the political subdivision by which they are issued.