The only requirement of form for simple contracts which can be said to exist independently of statute is in the case of negotiable bills of exchange and promissory notes. A bill of exchange is a kind of contract which originated in the custom of merchants, and which is designed to take the place of money, to some extent, as a circulating medium; and from its very nature and use as a negotiable instrument it must be in writing. The same may be said of promissory notes, which, whether by statute or the law merchant, are negotiable, and stand on the same footing as bills of exchange. Besides the mere necessity of writing, these instruments are required by law to be in a particular form, but this is matter more properly for a work on negotiable instruments.