Ind. Const. art. 10, § 2

All the revenues derived from the sale of any of the public works belonging to the State, and from the net annual income thereof, and any surplus that may, at any time, remain in the Treasury, derived from taxation for general State purposes, after the payment of the ordinary expenses of the government, and of the interest on bonds of the State, other than Bank bonds; shall be annually applied, under the direction of the General Assembly, to the payment of the principal of the Public Debt.