Where a suit is brought on a contract signed by an agent, and a letter authorizing the agent to make the contract has been lost, proof of its contents may be introduced without advertisement of its loss. The letter is not the document on which the action is founded.

2. The genuineness of the letter need not be proved by witnesses who have seen the writer write and sign her name; but any other evidence, even circumstantial facts going to show the existence and authenticity of the letter, are admissible.

3. When an obligation to do is actively violated, or when the obligor puts it out of his power to comply, no default is necessary.