Why did Tom and Huck go to the graveyard with a dead cat?

Huck is planning on entering a cemetery at midnight to perform a witch's ritual to cure warts. Tom and Huck discuss the merits of various superstitions and strange chants before they agree to meet later that night to go to the cemetery together.

Huck claims that a person can use a dead cat to cure warts. The trick is to take the dead cat to a graveyard, say a special incantation over the grave of a freshly dead evil man, and throw the cat after his spirit. ... Huck and Tom go to the graveyard to test the theory.