THE VIENNA CONVENTION, 1857
"Each state is at liberty to permit convention gold
coins to be paid into their offices instead of silver, according to a previously settled fixed rate, and to extend this permission either to all transactions and offices, or only to some. Such previous settlement of the rate is, however, never to last more than six months, and must at the expiration of the last month always be renewed for the following official treasury period of exchange. The rate cannot be fixed at a higher value than that given to such coinage by the average of the official commercial rate of exchange during the previous six months. Each government also reserves to itself the right to alter the rate at any time within the period fixed, and to suspend it when it thinks proper.
"A treasury rate of exchange shall henceforth only be fixed for convention gold coins, and not for other kinds of coined gold.
"The widest circulation to be given to the notices by which the official rate of exchange is fixed. They must be published beforehand, even when a change in rate for the next fixed period is not intended, and must contain—
"1. The statement of the average trade exchange at the principal places of exchange, during the six months immediately preceding.
"2. The treasury rate fixed accordingly.
"3. The duration of the value of the same.
"4. The reservation to alter or recall this rate of exchange if necessary, even before the expiration of the term named.