For the purpose of creating a sinking fund for the gradual extinguishment of the bonds and funded debt of the respective cities of this commonwealth, the councils of each thereof shall, annually (until payment of the bonds and funded debt be fully provided for) levy and collect, in addition to the other taxes of said corporation, a tax of not less than one mill and not exceeding three mills upon the assessed value of the taxable property of each of said cities, to be called the sinking fund tax, which shall be paid into the city treasury and shall be applied towards the extinguishment of said bonds and funded debt in the order of the date of issue thereof, and to no other purpose whatever.
53 P.S. § 25851