Dewar flask. An early design of vacuum flask made by James Dewar in 1898. This consists of two glass flasks, one inside the other, that are joined at the top. The space between the flasks is evacuated of air. Both flasks have silvered surfaces. The coating and the vacuum combine to drastically reduce heat transport by convection or conduction. Dewar used this design in his experiments on the magnetic properties of liquid hydrogen. This object is preserved at the Royal Institution, London.