British chemist and physicist whose study of low-temperature phenomena entailed the use of a double-walled vacuum flask of his own design which has been named for him.

Educated at the University of Edinburgh, Dewar became a professor at the University of Cambridge (1875) and at the Royal Institution of Great Britain, London (1877), holding both posts throughout his life.

Dewar developed structural formulas for benzene (1867), did research in spectroscopy for more than 25 years, and by 1891 had constructed a machine for producing liquid oxygen in quantity. About 1892 he conceived the idea of using vacuum-jacketed vessels for the storage of low-temperature liquid gases, and the resulting device proved so efficient in preventing the influx of external heat that it became an essential tool in low-temperature scientific work. The principle of the Dewar flask has also been used extensively in the common thermos bottle. Dewar was subsequently the first to liquefy hydrogen gas (1898) and to solidify it (1899). He was knighted in 1904. His discovery (1905) that cooled charcoal can be used to help create high vacuums later proved useful in atomic physics. With Sir Frederick Augustus Abel he developed cordite, an explosive.