"Money in the custody of the bank is to all intent and purposes the money of the banker, to do with it as he pleases; he is guilty of no breach of trust in employing it; he is not answerable to the customer if he puts it into risk speculation; he is not bound to keep it or deal with it as the property of the customer; but he is answerable for the amount because he has contracted to repay to the customer when demand, a sum equivalent to that paid into his hands."