Whenever the people of any county shall desire to erect, or contribute to the erection of a memorial building in honor of their soldiers and sailors and shall procure bona fide subscription to the extent of two-thirds of the estimated cost thereof, it shall be lawful for any county by a two-thirds vote of the county board or board of county commissioners, as the case may be, to contribute such sum or sums of money toward the cost of erecting such a memorial building not exceeding one-third of the cost thereof, as shall be deemed discreet and proper: Provided, that such contribution shall be made subject to the following conditions: (1) said building shall be constructed in accordance with plans, approved by such county board; (2) when erected, shall be forever used exclusively for public or civic purposes; and (3) if, at any time after the construction thereof, it shall cease to be used for such purposes, it and the property upon which it is situate, shall revert to and become the property of said county.