> What happens, then, if the supply of gold increases, demand for money remaining the same? The "price of money" falls, i.e., the purchasing power of the money-unit will fall all along the line. An ounce of gold will now be worth less than 100 loaves of bread, 1/3 of a television set, etc. Conversely, if the supply of gold falls, the purchasing power of the gold-ounce rises.

Gold and silver have worked well because the increase in supply has remained fairly constant.  It has grown organically in close parallel with human ability. 
 Modernity has brought a huge spike in ability to mine, however, and governments can 'find' pounds and pounds of previously undisclosed metals that were not already priced in.

This is one of the arguments for Bitcoin as money.  The supply is known and predictable and unchanging.  Exchanges can still operate without full reserves, however, when they don't provide the end user full auditability, similar to paper gold.  Tradeoffs.