S. 226, A bill to suppress the slave trade in the District of Columbia, September 3, 1850
This bill, passed by Congress on September 20, 1850, ended slave trading in Washington, D.C., a notorious slave market. It outlawed the transport of persons into the District for sale and liberated enslaved individuals brought into the city for that purpose. Slavery in the District continued, however, until Congress passed the D.C. Emancipation Act in 1862.
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it shall not be lawful to bring into the District of Columbia any slave whatever, for the purpose of being sold.
Records of the United States Senate, National Archives and Records Administration