A motion to postpone a trial on the ground of the absence of evidence can only be made upon affidavit showing the materiality of the evidence expected to be obtained, and that due diligence has been used to procure it. The court may require the moving party, where application is made on account of the absence of a material witness, to state upon affidavit the evidence which he expects to obtain; and if the adverse party thereupon admits that such evidence would be given, and that it be considered as actually given on the trial, or offered and overruled as improper, the trial must not be postponed.