theory of money how a deflation is possible

1. During the late nineteenth century, the United States experienced a period of sustained deflation, or a falling price level. Explain in terms of the quantity theory of money how a deflation is possible. Is it necessary for the quantity of money to decline for deflation to occur?
2. How does a high rate of inflation affect the value of money? How does it affect the usefulness of money as a medium of exchange?