Where a corporation has, before the commencement of the principal Act, been entered unconditionally in any lands, and no casualty can be exacted in respect of such lands during the existence of the corporation, it shall not be competent to the superior of such lands to give notice of redemption of casualties, or to exact compensation in respect of such redemption, unless, within fifteen years after the commencement of this Act, it shall become competent to exact a casualty in respect of such lands, failing which the lands, whether still belonging to such corporation or not, shall, on the expiry of the said period of fifteen years be deemed for the future to be discharged of all casualties and of all claims for compensation in respect thereof.