The Wealth of Nations by Part 1 Chapter 5 Page 34

One-and-twenty worn and defaced shillings, however, were considered as equivalent to a guinea, which perhaps, indeed, was worn and defaced too, but seldom so much so. The late regulations have brought the gold coin as near perhaps to its standard weight as it is possible to bring the current coin of any nation; and the order, to receive no gold at the public offices but by weight, is likely to preserve it so, as long as that order is enforced. The silver coin still continues in the same worn and degraded state as before the reformation of the gold coin. In the market, however, one-and-twenty shillings of this degraded silver coin are still considered as worth a guinea of this excellent gold coin.

The reformation of the gold coin has evidently raised the value of the silver coin which can be exchanged for it.