PYREX® Vista™ Short Neck Round Bottom Boiling Flasks have full length outer 24/40 glass joints, except 50ml size which has full length outer 19/38 joint. These round bottom boiling flasks are manufactured from No. 7740 borosilicate glass. Wall thicknesses are controlled to provide balance between thermal expansion and mechanical strength for greater shock resistance. These flasks are designed to ASTM E-1403, type II, class 3 —standard taper neck, short neck round bottomed glass boiling flasks.