That in all cities of which the mayor and alderpersons have heretofore been incorporated by any special act, as a cemetery association or body politic, it shall be lawful, a majority of their number assenting thereto, for such association or body politic to demise for a term of years, or to convey in perpetuity any real estate which it may have acquired by purchase or otherwise; and the real estate so conveyed shall be devoted exclusively for burial or cemetery purposes by the grantee or lessee thereof.