A motion to postpone a trial on account of the absence of evidence may be made only upon affidavit showing the materiality of the evidence expected to be obtained, and that due diligence has been used to obtain it. If the motion is based on the absence of a witness, the affidavit must show what facts the affiant believes the witness will prove, and not merely the effect of such facts in evidence, and that the affiant believes them to be true. If the adverse party will consent that, on the trial, the affidavit may be read as the deposition of the absent witness, the trial shall not be postponed on account of his absence.