Atticus is trying to explain to Jem why the jury decided to convict Tom Robinson. He tells Jem,
"Tom Robinson's a colored man, Jem. No jury in this part of the world's going to say ' We think your're guilty, but not very,' on a charge like that. It was either a straight acquittal or nothing." (pg 218)
Jem then thinks that maybe rape should not be a capital offense and punishable by death. But Atticus explains,
"He didn't have any quarrel with the rape statute, none whatsoever, but he did have deep misgivings when the state asked for and the jury gave the death penalty on purely circunstantial evidence... There is always the possibillity, no matter how improbable, that he's (the accused) innocent" (pg 219-220)
Jem thinks then that it is all up to the jury, and since every man brings with him to the jury box his beliefs about black men and white men, then juries cannot make wise decisions and should not be given that responsibility.
Atticus suggests that a better way to handle the situation would be to change the law.
"Change it so that only judges have the power of fixing the penalty in capital cases." (pg 220)
Jem tells Atticus that he should go to Montgomery and change the law. Remember that Atticus represents Maycomb in the state legislature. Atticus tells him that it is more complicated then that. Atticus responds that
"You'd be surprised how hard that'd be. I won't live to see the law changed, and if you live to see it you'll be an old man." (pg 220)
Atticus also tries to instill some moral ideas in Jem when he tells him,
"As you grow older, you'll see white men cheat black men every day of your life, but let me tell you something and don't you forget it --- whenever a white man does that to a black man, no matter who is he, how rich he is, or how fine a family he comes from, the white man is trash." (pg 220)
All quotes are from my edition of the book. The page numbers may differ in your book, but they should be close.