(70) After any bill filed, and before the defendant hath answered the same, upon affidavit made that any of the plaintiff's witnesses are aged and infirm, or going out of the country, or that any of them is a single witness to a material fact, the clerk of the court shall, as of course, upon the application of the plaintiff, issue a commission to such commissioner or commissioners, as the judge of the court may direct, to take the examination of such witness or witnesses de bene esse, upon giving due notice to the adverse party of the time and place of taking his testimony.