In Chapter 10, Huck plays a joke on Jim because Jim believes that touching a snake skin brings bad luck. Huck doesn't believe in it, and puts a dead rattlesnake on Jim's bed to prove his point. It does bring bad luck, however, when the snake's mate comes and curls around the dead snake and bites Jim in the heel.
Jim is sick for four days after the snake bite, and Huck feels terrible about what he has done. This scene shows that Huck is becoming increasingly more mature and more concerned about Jim.