The Wealth of Nations by Part 4 Chapter 3 Page 39

whole value which in this state of things could justly be supposed due to them.

Even in ordinary and quiet times it is the interest of the holders of receipts to depress the agio, in order either to buy bank money (and consequently the bullion, which their receipts would then enable them to take out of the bank) so much cheaper, or to sell their receipts to those who have bank money, and who want to take out bullion, so much dearer; the price of a receipt being generally equal to the difference between the market price of bank money, and that of the coin or bullion for which the receipt had been granted. It is the interest of the owners of bank money, on the contrary, to raise the agio, in order either to sell their bank money so much dearer, or to buy a receipt so much cheaper.