There are many great resources available on the Internet... here are a few...
http://www.aip.org/history/electron/
http://www.cs.uh.edu/~clifton/macro.l.html
http://www.libraryspot.com/encyclopedias.htm
Making of America: http://cdl.library.cornell.edu/moa/
We the People: http://www.eduplace.com/links/wtp/grade6.html
http://www.allmeasures.com/Formulae/static/materials/31/density.htm