APS Online contains full digital images of over 1,100 periodicals spanning nearly 200 years-from colonial times to the advent of American involvement in World War II (1940).
Accessible Archives also offers primary source material from 18th and 19th century periodicals.
MLA International Bibliography and other databases relevant to your research: Academic Search Complete and America: History and Life
ProjectMuse has full-text scholarly articles from many journals in the humanities
JSTOR has full-text scholarly articles from journals in many disciplines.