Bonds which may be issued for building construction and improvement purposes at Alabama State Hospitals and at Partlow State School for Mental Deficients, under the provisions of the amendment to the Constitution which was proposed by Act No. 37 adopted at the First Special Session of the Legislature of 1956 [1956, 1st Ex. Sess., p. 63] and which was ratified by the electors at the general election held on November 6, 1956 [amendment No. 113], may be sold, executed and delivered from time to time in series, may bear such date or dates, and may bear interest at such rate or rates not exceeding four and one-half per centum per annum payable semiannually and evidenced in such manner, all as may be provided at the respective times of the sales thereof. The maturities of the bonds of each series shall, to such extent as may be practicable, be so arranged that the first maturity of the bonds of each series shall be not later than one year after the date thereof and the last maturity of the bonds of that series shall be not later than twenty years after the date thereof, and the said maturities shall be so arranged that during each then succeeding fiscal year of the state the aggregate installments of principal and interest that will mature on all bonds that will be outstanding under said amendment, immediately following the issuance of the bonds of that series, will be substantially equal; provided, that the determination in the proceedings under which the bonds are issued that the requirements of this sentence have been complied with shall be conclusive of such compliance and the purchasers of any of the bonds and all subsequent holders thereof shall be fully protected by such determination.