On the trial of any indictment or information for a defamatory libel, the defendant having pleaded such plea as hereinafter mentioned, the truth of the matters charged may be inquired into, but shall not amount to a defence, unless it was for the public benefit that the said matters charged should be published; and to entitle the defendant to give evidence of the truth of such matters charged as a defence to such indictment or information it shall be necessary for the defendant, in pleading to the said indictment or information, to allege the truth of the said matters charged in the manner now required in pleading a justification to an action for defamation, and further to allege that it was for the public benefit that the said matters charged should be published, and the particular fact or facts by reason whereof it was for the public benefit that the said matters charged should be published, to which plea the prosecutor shall be at liberty to reply generally, denying the whole thereof; and if after such plea the defendant shall be convicted on such indictment or information it shall be competent to the court, in pronouncing sentence, to consider whether the guilt of the defendant is aggravated or mitigated by the said plea, and by the evidence given to prove or to disprove the same: Provided always, that the truth of the matters charged in the alleged libel complained of by such indictment or information shall in no case be inquired into without such plea of justification: Provided also, that in addition to such plea it shall be competent to the defendant to plead a plea of not guilty: Provided also, that nothing in this Act contained shall take away or prejudice any defence under the plea of not guilty which it is now competent to the defendant to make under such plea to any action or indictment or information for defamatory words or libel.