Powers of all public officers are defined by law and all persons must take notice thereof. The public may not be estopped by the acts of any officer done in the exercise of an unconferred power. (Civil Code 1895, § 268; Civil Code 1910, § 303; Code 1933, § 89-903.) History of Code section. — This Code section is derived from the decision in Penitentiary Co. No. 2 v. Gordon, 85 Ga. 159, 11 S.E. 584 (1890).