It has a hole in the top where a BÃ 1\/4 chner funnel can be put and a small tube in the side where a vacuum can be attached . It is primarily used together with a BÃ 1\/4 chner funnel fitted through a drilled rubber bung or an elastomer adapter ( a BÃ 1\/4 chner ring ) at the neck on top of the flask for filtration of samples . The BÃ 1\/4 chner funnel holds the sample isolated from the suction by a layer of filter paper .