The state of Alabama is hereby authorized to issue not exceeding $900,000 aggregate principal amount of bonds for the purpose of paying or retiring prior to maturity, the bonds of Alabama bridge commission (an agency of the state of Alabama) which were outstanding on July 1, 1939. Said bonds shall be general obligations of the state of Alabama, to the prompt payment of the principal of and interest on which the full faith and credit and taxing power of the state are hereby irrevocably pledged, and all of said bonds, together with any other bonds of the state providing for a pledge of said gasoline excise tax which may be authorized by constitutional amendment ratified on the same day as this amendment is ratified shall be additionally secured, without priority of one bond over another, by a pledge of the proceeds of the gasoline excise tax, authorized to be pledged to the highway bonds provided for in the amendment to the Constitution known as article XXA [Amendment No. 21], subject, however, to the prior pledge of said gasoline tax to said highway bonds. The bonds hereby authorized shall bear interest at not exceeding three per centum (3%) per annum, payable semi-annually, and shall be sold at not less than the par value thereof. Said bonds and the interest thereon shall be forever exempt from taxes of every kind. Said bonds shall be issued at such time or times, in such denominations and series and shall mature at such times, not later, however, than fifteen (15) years from the date of issuance, and shall have such other terms and conditions as may be provided by law.