Every person, firm or corporation employing females in any factory, mercantile establishment, mill or workshop in this state shall provide a reasonable number of suitable seats for the use of such female employees, and shall permit the use of such seats by them when they are not necessarily engaged in active duties for which they are employed, and shall permit the use of such seats at all times when such use would not actually and necessarily interfere with the proper discharge of the duties of such employees, and, where practicable, such seats shall be made permanent fixtures and may be so constructed or adjusted that, when not in use, they will not obstruct such female employee when engaged in the performance of her duties.