Principals are capable of delegating authority to others to act in their behalf and for their interests. In general, whenever a person has power to do a thing he may do it by an agent, and every person of full age, free from disabilities, has complete capacity for this purpose; but infants, in some States - to some extent - married women, idiots, lunatics, and other persons not sui juris, are either wholly or partially incapable of appointing agents. An infant may authorize another to do an act which is for his benefit, but he can not authorize him to do an act which is to his prejudice. Story on Agency, Sec. 6; Mechem on Agency,

Secs. 18, 43, 47, 54, 56.