vessel with double walls, the space between which is evacuated. It was invented by the British chemist and physicist Sir James Dewar (Dewar, Sir James) in the 1890s. Thermos is a proprietary name applied to a form protected by a metal casing.

The vacuum flask was devised to preserve liquefied gases by preventing the transfer of heat from the surroundings to the liquid. The evacuated space between the walls (which are ordinarily glass or steel) is practically a nonconductor of heat; radiation is reduced to a minimum by silvering the glass or steel. The chief path by which heat can be communicated to the interior of the inner vessel is at the vessel's neck, the only junction of the walls, which therefore is made as small as possible. This thermal isolation applies equally to heat, a hot liquid remaining at a high temperature in the flask for several hours.