The interest accrued, earned or paid as the result of investment of said endowment fund is hereby appropriated to said Department of Archives and History and shall be used by said department only for such purposes as its trustees may specify and the Governor approve; provided, however, that no expenditure of such funds may be made or approved by said board of trustees unless it is for the purpose of acquiring rare and valuable articles, property or materials or acquiring, marking and preserving or maintaining historical locations or spots within the State of Alabama.