On every private act of assembly heretofore passed, or which shall hereafter be passed, and which shall be subject to enrollment tax as herein provided, said tax shall be paid within one year after the approval thereof by the governor, or his failure to return the same to the legislature as required by the last clause of the twenty-third section of the first article of the constitution; and in default of payment as aforesaid, every such enactment shall thereafter be null and void; and the time for paying the enrollment tax upon all acts heretofore passed, upon which the enrollment tax has not been paid, is hereby extended for the period of one year from the passage of this act.