In the experiment conducted by Joseph priestley which was using a candle and plant he would need to rekindle the candle to test whether it burns after a few days.

so i was wondering how the candle could be lighted without disturbing the setup?

its a humble request, so do answer my question.

Even my bio teacher could not answer it!

eagerly waiting for reply. thanku.

Without disturbing it's not possible. The candle is reburned and a plant is kept in the next reaction. eventhough some O2 will enter that time it won't alter the result of the reaction bcoz  the candle consumes it within minuts. The imp. is for the placed plant that it gives out O2 to keep candle always burned. The aim of this exeperiment is to prove that plant purifies the air which was damaged by burning.