Where no other instruction defining a broker's duty is given, it is error to refuse an instruction that the duty of a broker is to bring the buyer and seller together and effect a purchase of the property according to the terms agreed on by the seller and the broker, and that the latter is not entitled to a commission for an unsuccessful effort to effect a sale. West v. Demme, 128 Mich. 11, 87 N. W. 95. Compare Sec. 509.