One who has given a broker authority until further notice to sell land has the burden of proving that he revoked the authority before the broker found a purchaser. Bourke v. Van Keuren, 20 Colo. 95, 36 P. 882; Clements v. Stapleton, 136 Iowa, 137, 113 N. W. 546; Bradley v. Blandin, 100 A. 920, 91 Vt. 472.