Disks The use of disks is permitted only where specified or allowed in the monograph lf specified in the individual monograph each tube is provided with a cylindrical disk 9.5 ± 0.15 mm thick and 20.7 ± 0.15 mm in diameter The disks is made of a suitable transparent plastic material having a specific gravity of between 1.18 and 1.20. Five parallel 2 ± 0.1 –mm holes extend between the ends of the cylinder. One of the holes is centered on the cylindrical axis. The other holes are centered 6 ± 0.2 mm from the axis on imaginary lines perpendicular to the axis and parallel to each other Four identical trapezoidal-shaped planes are cut into the wall of the cylinder nearly perpendicular to the ends of the cylinder The trapezoidal shape is symmetrical its parallel sides coincide with the ends of the cylinder and are parallel to an imaginary line connecting the centers of two adjacent holes 6 mm from the cylindrical axis. The parallel side of the trapezoid on the bottom of the cylinder has a length of 1.6 ± 0.1 mm and its bottom edges lie at a depth of 1.5 to 1.8 mm from the cylinder’s circumference. The parallel of 9.4 ± 0.2 mm. and its center lies at a depth of 2.6 ± 0.1 mm from the cylinder’s circumference. All surfaces of the disk are smooth. Lf the use of disks is specified in the individual monograph add a disk to each tube and operate the apparatus as directed under Procedure The disks conform to dimensions found in Figure1