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In form, a Dewar flask is a glass or metal bottle, with a double-layer construction; in fact, it can be considered to be two thin-walled bottles nested one inside the other, and sealed together at the neck. The narrow space between is evacuated almost entirely of air; the near vacuum prevents conduction and convection of heat.

The inner surface of the outer bottle, and the outer surface of the inner bottle, have a metallic or similar reflective coating to prevent heat from being transmitted via radiation. This is so effective that what little heat transport does occur is almost entirely via the neck and stopper; insulating materials such as cork are often used for the stopper.

A very common use of the Dewar flask in laboratories is the storage of liquid nitrogen; in this case, the leakage of heat into the extremely cold interior of the bottle results in a slow "boiling-off" of the liquid (a pressure relief valvePressure relief valves are most commonly used with high pressure operating systems. Steam boilers usually exceeed to higher pressure limitations of the equipment, which can be controlled back with the help of pressure relief valves. The pressure relief va is provided to prevent pressurePressure (symbol: p is a measure of force per unit area. where p is the pressure F is the force A is the area Often F is taken to be the of the magnitude of the mean vector force normal to the surface of area A upon which it exerts; the "surface" not nece from building up). The excellent insulation of the Dewar flask results in a very slow "boil" and thus the nitrogen lasts a long time without the need for expensive refrigerationRefrigeration (from the Latin frigus frost) is generally the cooling of a body by the transfer of a portion of its heat away from it. Applications include conservation, especially of food, and lowering the temperature of drinks to one that is more agreeab equipment.

A "Thermos bottle" ( trademarkSee also: brand Bass Red Triangle, was the first trademark registered in Britain in 1876. A trademark ( Commonwealth English: trade mark is a distinctive name, phrase, symbol, design, picture, or style used by a business to identify itself and its producted), or vacuum flaskLunchbox and vacuum bottle owned by Harry S. Truman A vacuum flask or Thermos flask is a bottle that reduces heat transfer from the inside to the outside and conversely to a minimum, and therefore keeps warm drinks warm and refrigerated drinks cold. It wa (generic) used to keep coffeeThis article discusses the coffee plant; for information on the beverage see coffee (drink). Coffea arabica Arabian Coffee Coffea benghalensis Bengal Coffee Coffea canephora Robusta Coffee Coffea congensis Congo Coffee Coffea liberica Liberian Coffee Coff or other beverages hot, is usually based on the design of a Dewar flask; since glass dewar flasks are fragile and somewhat expensive to fabricate, other constructions based on plasticThe term plastics covers a range of synthetic or semi-synthetic organic condensation or polymerization products that can be molded or extruded into objects or films or fibers. Their name is derived from the fact that in their semi-liquid state they are ma or insulated metal are also popular.