This section is from the book "Handbook Of The Law Of Contracts", by Wm. L. Clark, Jr.. Also available from Amazon: Handbook of the law of contracts.
125. A corporation, by reason of its artificial nature, can only contract through a duly-authorized agent.
126. Formerly, with certain exceptions, it could only contract under its corporate seal; but now, unless restricted by its charter or by statute, it may contract in the same manner as a natural person.
127. The power of a corporation to enter into a contract is limited in respect of the subject-matter only by its charter or act of incorporation or by other statutes binding on it. Except as so restricted, it has the implied power to enter into any contract which is reasonably incidental to the accomplishment of the objects for which it is created.
128. An attempted contract, which is not within the powers of a corporation, is said to be ultra vires, and in many jurisdictions is held to be void, so that it cannot be enforced; but in other jurisdictions the defense of ultra vires is excluded when the contract has been performed by the party seeking to enforce it, and it would be inequitable to allow the defense.
35 Picard v. Hire, 5 Ch. App. 277. Bee "Eusband and Wife" Dec. Dig. (Key-Xo.) § 215; Cent. Dig. § 784.
A corporation can contract only by means of an agent. It "cannot act in its own person, for it has no person."86 It cannot act through one or any number of its members, merely as such, for, though they compose the corporation, they are not the corporation. It must act through an agent expressly authorized to act for it.87
 
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