They had slaves in Philadelphia? I also thought Washington freed all his slaves after the war?
http://www.mountvernon.org/learn/meet_george/index.cfm/ss/101/
Slavery was legal in most of the 13 states in the 1700s. It was abolished in all of the Northern States, with the last state, New York, freeing all slaves by 1826.
New York took so long because there were so many slaves in New York. They numbered in the tens of thousands. I believe at the time New York voted in abolition (which was 1801 or so) they had more slaves than any other state except Virginia. New York phased out slavery over a 25-year period as I recall.
Virginia almost passed a similar law in the 1790s. Shame it did not pass. Would have changed American history significantly.