There are two ways to search for newspaper and magazine articles. One way is to know the date in question and look by the date in older magazines and newspapers. Older magazines are available in alphabetical order by title on the 3rd floor in the Periodicals area. Example titles include, Time, Life, Newsweek, Contemporary Review, Blackwood's Magazine, etc. Older newspaper are available only in microfilm on the 4th floor behind the old elevators. Example titles include, New York Times, Los Angeles Times, Wall Street Journal, Christian Science Monitor, etc.
Historic Newspapers (guide to finding them)
Newspapers from the last 20 Years (these are available online through several databases)
For magazines you don’t know or can’t browse, look in a periodical index. Look up your topic (in this case, slavery) and it will tell you which magazine to use, what year, volume, issue, and page you will find the article. You may then go to it or interlibrary loan (ILL) it. Below are some online indexes you might consult.