This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
Military officers are not subject to impeachment. No attempt has ever been made to impeach an officer of the army or of the navy, and, therefore, there have been no pronouncements upon this point. But there has been no question as to this doctrine.
Members of Congress are not officers of the United States, not being commissioned by the President. This point was made at the time of the impeachment of Senator Blount, a resolution to the effect that he was an officer being negatived by a vote of fourteen to eleven.