) ▁until ▁the ▁mid - 1 8 th ▁century , ▁although ▁they ▁emer ged ▁as ▁vocal ▁oppon ents ▁of ▁sla very ▁in ▁the ▁Revolution ary ▁era . ▁ ▁Further ▁events ▁ ▁Late ▁ 1 8 th ▁and ▁ 1 9 th ▁century ▁During ▁and ▁following ▁the ▁Revolution , ▁the ▁northern ▁states ▁all ▁abol ished ▁sla very , ▁with ▁New ▁Jersey ▁acting ▁last ▁in ▁ 1 8 0 4 . ▁Some ▁of ▁these ▁state ▁juris dict ions ▁en act ed ▁the ▁first ▁abol ition ▁laws ▁in ▁the ▁entire ▁New ▁World . ▁ ▁In ▁states ▁that ▁passed ▁grad ual ▁abol ition ▁laws , ▁such ▁as ▁New ▁York ▁and ▁New ▁Jersey , ▁children ▁born ▁to ▁slave ▁moth ers ▁had ▁to ▁serve ▁an ▁extended ▁period ▁of ▁indent ure ▁into ▁young ▁ad ul th ood . ▁In ▁other ▁cases , ▁some ▁slaves ▁were ▁rec lass ified ▁as ▁indent ured ▁servants , ▁effectively ▁pres erving ▁the ▁institution ▁of ▁sla very ▁through ▁another ▁name . ▁ ▁O ften ▁cit ing ▁Revolution ary ▁ide als , ▁some ▁slave hold ers ▁fre ed ▁their ▁slaves ▁in ▁the ▁first ▁two ▁dec ades ▁after ▁independence , ▁either ▁out right ▁or ▁through ▁their ▁will s . ▁The ▁proportion ▁of ▁free ▁black s ▁rose ▁marked ly ▁in ▁the ▁Upper ▁South ▁in ▁this ▁period , ▁before ▁the ▁in vention ▁of ▁the ▁cot ton ▁g in ▁created ▁a ▁new ▁demand ▁for ▁slaves ▁in ▁the ▁developing ▁" C ot ton ▁Kingdom " ▁of ▁the ▁Deep ▁South . ▁ ▁By ▁ 1 8 0 8 ▁( the ▁first ▁year ▁allowed