PYREX® 75mm diameter ribbed watch glasses are designed for use as covers for 250ml PYREX® beakers. Ribs are molded into the glass so that they can't pop off as they do when cane is sealed to standard watch glasses. Permits escape of excess vapor and eliminates risk of contamination from dropped hooks or props. Meets EPA requirements for methods 3005, 3010, and 3020. Material: Borosilicate Glass Code 7740. Glass Style: Ribbed. Top Style: Heavy Duty Rim. Fits Griffin Beaker Size: 250mL. Diameter: 75mm (Approx). Reusable. 12/Case. Corning #: 9990-75.