A Dewar flask is a glass vessel used for keeping liquids at temperatures differing from that of the surrounding air. This is done by reducing to a minimum the transfer of heat between the liquid and the air. A dewar flask consists of a double-walled flask, with the space between the two walls exhausted to a very high vacuum, to minimize transfer of heat by convection and conduction. The inner surfaces of the walls are silvered to reduce transfer of heat by radiation; areas of contact between the two walls are kept at a minimum to keep down conduction of heat.