Agent's proprietary interest prevents unilateral revocation of authority where revocation would prejudice that interest absent express agreement. Where an agent possesses a proprietary or secured interest in the agency's subject-matter, the principal cannot revoke or terminate the agency to the prejudice of that interest in the absence of an express agreement; this protection applies even if the principal becomes insane or dies, and is exemplified by authority to sell property or consignments where the agent may repay advances from proceeds.