official oath. An assessor who fails to take the general oath of office required bythelaw, is an officer de facto, and his acts are valid when questioned collaterally. Moore v. Turner, 43 Ark., 243; Twombly v. Kimbrough, 24 ib., 474; Equalization Board v. Landowners, etc., 51 ib., 516; Cooley Taxation [2 ed.), PP. 253-6.