What theory states that a nation could increase its wealth and power by obtaining as much gold and silver as possible?

This theory is the Gold Standard and the Silver Standard. This is a good way of boosting the worth of a particular country's currency. There was an economical crisis in 1857 which forced America to create a single currency-- in 1900, America passed the Gold Standard Act, and in 1933, President Franklin D. Roosevelt made owning gold illegal (above a small amount).