Notwithstanding any prohibitory, irritant, and resolutive clauses contained in any entails already made and established, or which may hereafter be made and established, pursuant to the directions of the Entail Act 1685, it shall be lawful for the respective heirs of entail in possession to grant tacks of any parts of the lands, estates, or heritages therein contained, for the fair rent of such lands or heritages at the period of letting, either by public auction or private bargain, and notwithstanding any prohibition against diminution of the rental, for any period not exceeding twenty-one years, and to grant tacks of any mines and minerals contained in such lands and estates for any period not exceeding thirty-one years: Provided always, that nothing herein contained shall authorize any heir of entail in possession of any entailed lands, estates, or heritages, to take any grassum or valuable consideration, other than the tack duty or rent, for granting any tack, or to grant any tack of the home farm, nor of the mansion house and offices, or of the garden, lawn, park, or policy attached thereto, for any period beyond his own life; and in case any such grassum or consideration shall be taken, or in case any tack hereby prohibited shall be granted, such tack shall be null and void.