The board of county commissioners is hereby authorized and empowered in aid of the powers granted or prescribed in RCW 8.08.090 to levy, annually, a tax as large as may be necessary, but not exceeding the rate of one mill on the dollar, upon all the taxable property in the county, such tax to be assessed, levied and collected at the same time and in the same manner as taxes for general county purposes, but the proceeds of said taxes, when collected, shall constitute and be a special fund, applicable solely to the cost of such condemnation, appropriation or disposition, as is mentioned in RCW 8.08.090, and the expenses incident thereto.