a "snyder column" is an extremely effective air-cooled column used in selected fractional distillations. it is a single glass tube with a series of circular indentations or restrictions in the walls of the cylinder (often 3 or 6) in which rest, inverted, the same number of roughly tear-shaped, hollow, sealed glass stoppers; above each point where an inverted tear-shaped stopper rests, the cylinder has further vigreux-type indentations, in this case serving to limit how high the glass stopper can be raised (by vapor flow) above its resting place, where, when not raised, it seals the circular opening created by the circular indentation. these floating glass stoppers act as check valves, closing and opening with vapor flow, and enhancing vapor-condensate mixing. a standard application of snyder columns, is as the condenser and fractionation column above a kuderna-danish concentrator, used to efficiently separate a low boiling extraction solvent such as methylene chloride from volatile but higher boiling extract components (e.g., after the extraction of organic contaminants in soil).