Congress may alter, amend, add to, or repeal any of the provisions of sections 41 to 46, 48, 50, 51 to 53, and 54 to 57, of this title; but no contract or individual right made or acquired under such provisions shall be thereby divested or impaired.
(R.S. §5594.)
R.S. §5594 derived from act Aug. 10, 1846, ch. 178, §11,