18. A witness shall not be excused from answering any question put to him on the ground that the answer to such question will criminate or may tend directly or indirectly to criminate such witness, or that it will expose or tend directly or indirectly to expose such witness to a penalty or forfeiture of any kind: Provided that no such answer which a witness shall be compelled to give shall subject him to any arrest or prosecution or be proved against him in any criminal proceeding, except a prosecution for giving false evidence.