The ‘Thermos Flask’ is a flask in which loss or gain of heat through conduction, convection and radiation has been reduced to a minimum. It is used for keeping a hot liquid hot and a cold liquid cold for a good length of time. The thermos flask consists of a double-walled glass flask placed on a spring within a metal casing, its mouth being closed by a cork stopper. The space between the walls is evacuated. The outer side of the inner wall and the inner side of the outer wall are silvered. The space between the vessel and the metal case is packed with felt or cork. The vaccum between the two walls of the glass bottle and the felt pad between the bottle and the metal case prevents the flow of heat due to conduction and convection. The silvered surface facing each other reduce radiation and absorption of heat to a minimum. Hence, heat can neither pass from the bottle to outside nor from outside into the bottle. Thus, a hot liquid placed in the bottle will remain hot and cold liquid cold for a sufficiently long time.