This apparatus will be best understood by a reference to the figures. The bell-jar A (fig. 1) rests, during filtration, on a square block of hard wood c, 25 cm. square by 25 mm. thick, over which is laid a sheet of vulcanised rubber d of good quality, and not less than 3 mm. thick. In the centre is a circular disk e also of hard wood, 13 cm. in diameter and about 20 mm. thick, which is held in position by four strong brass screws which pass down through the rubber into the square block below.