What does this quote mean from To Kill a Mockingbird?
I remembered something Jem had once explained to me when he went through a brief period of psychical research: he said if enough people—a stadium full, maybe—were to concentrate on one thing, such as setting a tree afire in the woods, that the tree would ignite of its own accord.
This quote, pulled from Chapter 21 in which Jem is yet convinced of the reasonableness of people and the power of human righteousness, conveys his naivete regarding the integrity of juries.
When he talks to Reverend Sykes, Jem sounds convinced that the jury will be fair and acquit Tom Robinson. But Reverend Sykes reminds Jem that white men are not likely to take the words of a black man over those of a white man, regardless of anyone's position. Still Jem argues that consensual acts of a sexual nature are not rape unless the victim is eighteen or under, and Mayella is nineteen. Moreover, she does not seem to have "kicked and hollered" much, either, for having been forced into an act she did not desire.
While the audience in the courtroom waits for hours on the verdict, Scout recalls what Jem once said when he was in a stage of his life in which he studied the powers of the human mind. Jem has explained to Scout the phenomenon of people concentrating upon an action desired, and this action occurs from people's having willed it. Scout wistfully imagines that all the audience, having heard the testimony of the witnesses, should work together and concentrate on a "not guilty" verdict for Tom because he is innocent. And, this knowledge of Tom's innocence should "ignite of its own accord."