What is French gold?

From the text:

“Your French gold?”
“Yes. We had occasion some months ago to strengthen our resources and borrowed for that purpose 30,000 napoleons from the Bank of France. It has become known that we have never had occasion to unpack the money, and that it is still lying in our cellar. The crate upon which I sit contains 2,000 napoleons packed between layers of lead foil. Our reserve of bullion is much larger at present than is usually kept in a single branch office, and the directors have had misgivings upon the subject.”