Sir James Dewar (20 September 1842 – 27 March 1923) was a Scottish chemist and physicist. He is probably best known today for his invention of the vacuum flask, which he used in conjunction with extensive research into the liquefaction of gases. He was also particularly interested in atomic and molecular spectroscopy, working in these fields for more than 25 years.

His name is most widely known in connection with his work on the liquefaction of the so-called permanent gases and his researches at temperatures approaching absolute zero. His interest in this branch of physics and chemistry dates back at least as far as 1874, when he discussed the "Latent Heat of Liquid Gases" before the British Association.

About 1892, the idea occurred to him of using vacuum-jacketed vessels for the storage of liquid gases. The Dewar flask (otherwise known as a Thermos or vacuum flask) was the invention for which he became famous for inventing. The vacuum flask was so efficient at keeping heat out, it was found possible to preserve the liquids for comparatively long periods, making examination of their optical properties possible. Dewar did not profit from the widespread adoption of his vacuum flask as he lost a court case against Thermos concerning the patent for his invention. While Dewar was recognised as the inventor, because he did not patent his invention, he was unable to stop Thermos from using the design.

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