The general assembly shall have no power to contract or authorize the contracting of any liability of the state, or to issue bonds therefor, except (1) to refund outstanding bonds, the refunding bonds to mature not more than twenty-five years from date, (2) on the recommendation of the governor, for a temporary liability to be incurred by reason of unforeseen emergency or casual deficiency in revenue, in a sum not to exceed one million dollars for any one year and to be paid in not more than five years from its creation, and (3) when the liability exceeds one million dollars, the general assembly as on constitutional amendments, or the people by the initiative, may also submit a measure containing the amount, purpose and terms of the liability, and if the measure is approved by a majority of the qualified electors of the state voting thereon at the election, the liability may be incurred, and the bonds issued therefor must be retired serially and by installments within a period not exceeding twenty-five years from their date. Before any bonds are issued under this section the general assembly shall make adequate provision for the payment of the principal and interest, and may provide an annual tax on all taxable property in an amount sufficient for the purpose.