Rubber bung

A laboratory rubber stopper or a rubber bung is mainly used in chemical laboratory in combination with flasks and test tube and also for fermentation in winery. Generally, in laboratory, the sizes of rubber stopper can be varied up to approximately 16 sizes and each of it is specific to certain type of container. As the rubber stopper is used in many experiment, some specific experiment requires a specific material. For example, the M35 Green Neoprene is for chemical resistance. For food fermentation, M18 white natural gum is preferred. For high temperature application, red or white silicone rubber stopper should be used.

Rubber bung can have one or more hole(s) for plugging in tube depending on the specification of the procedures. To prevent the liquid chemical leaks or escape the container, the rubber bung should fit tightly to the container's opening; therefore, the dimension of the rubber bung is of concern. These are some of the sizes that are commonly seen in chemical laboratory.

2000ml Erlenmeyer Flasks/ Florence Flask

Rubber bung size no.5 fits tightly on 50ml Pyrex® Flask

Other equipments such as glass tubes can be plugged into the holes of the rubber stopper

...
Wikipedia