The Wealth of Nations by Part 4 Chapter 3 Page 32

money equal to the price at which the bullion had been received. If he has no bank money of his own, he must purchase it of those who have it. The owner of bank money cannot draw out bullion without producing to the bank receipts for the quantity which he wants. If he has none of his own, he must buy them of those who have them. The holder of a receipt, when he purchases bank money, purchases the power of taking out a quantity of bullion, of which the mint price is five per cent above the bank price.

The agio of five per cent therefore, which he commonly pays for it, is paid not for an imaginary but for a real value. The owner of bank money, when he purchases a receipt, purchases the power of taking out a quantity of bullion of which the market price is commonly from two to three per cent above the mint price.