These Rubber Bulb Glass Droppers are Great to replace or add to a 2 Oz Boston Round Bottle. these glass droppers are wonderful, and dispensing liquids drop by drop gives wonderful control, and measuring. A wonderful way to make your own oil blends and perfumes too! Serums, carrier oils, almost any liquid will work in these droppers. 2/400 thread cap. Screw on cap THESE ARE PREMIUM GLASS PIPETTE DROPPERS.