Abstract: In a flask comprising a closed vessel with double walls enclosing a vacuum, layers of substances capable of absorbing light-rays and/or of selectively acting upon light-rays are applied to enable more radiant energy to reach the space enclosed by the inner wall than that space can radiate to the outside. Thus an increasing amount of heat is supplied to the space enclosed by the inner wall, and cold matter introduced into that space is heated. The effect of the light-rays can be increased by designing part of a protective jacket of the flask so that it is capable of being swung open and of acting as a reflector in that position.