AIR POROUS, SHEETLIKE FILTER MEDIA IN THE FORMS (1) OF PREFABRICATED POCKETS OF WOVEN GLASS AND CLOTH FOR MECHANICAL DRY TYPE DUST ARRESTORS OR COLLECTORS OF THE VIBRATING POCKET TYPE; AND (2) OF PRECUT-TO-SIZE SHEETS OF (A) FIBER GLASS, PAPER AND CLOTH FOR MECHANICAL DRY-TYPE DUST ARRESTORS OR COLLECTORS OF THE VIBRATING POCKET TYPE, (B) FIBER GLASS, PAPER, SYNTHETIC FIBERS AND COTTON WADDING FOR MECHANICAL DRY-TYPE VENTILATING AIR FILTERS OF THE PORTABLE CELL, FOLDABLE FRAME AND STATIONARY POCKET TYPES, AND (C) FIBER GLASS AND PAPER HAVING DIELECTRIC PROPERTIES FOR ELECTRICAL VENTILATING AIR CLEANERS OF THE STATIONARY DRY CELL TYPE AND THE DOMESTIC HOUSEHOLD TYPE