Place: United Kingdom, Scotland
Subject: biography, physics
Scottish physicist and chemist with great experimental skills that enabled him to carry out pioneering work on cryogenics, the properties of matter at extreme low temperatures. Dewar is also remembered for his invention of the Dewar vacuum flask, which has been adapted for everyday use as the thermos flask.
Dewar was born in Kincardine-on-Forth on 20 September 1842. He went to local schools until he contracted rheumatic fever in 1852, which required a long convalescence at home. He was nevertheless able to enter the University of Edinburgh in 1859, where he studied physical science and later worked as a demonstrator. In 1867, Dewar presented models of various chemical structures at an exhibition of the Edinburgh Royal Society. This work interested Friedrich Kekulé, who invited Dewar to Ghent in Belgium for the summer. Dewar then returned to Edinburgh, and continued as an assistant until 1873, holding a concurrent post as a lecturer at the Royal Veterinary College from 1869. The Jacksonian Professorship of Natural Experimental Philosophy and Chemistry at the University of Cambridge was offered to Dewar in 1875, followed two years later by the Fullerian Professorship of Chemistry at the Royal Institute in London. Dewar occupied both these chairs until his death. In 1877, he was also elected a Fellow of the Royal Society, which later awarded him the Rumford Medal for his research on the cryogenic properties of matter. Dewar was appointed to a government committee on explosives 1888-91, and in 1889 invented the important explosive cordite in collaboration with Frederick Abel (1827-1902). Dewar was knighted in 1904 and died in London on 27 March 1923.
Dewar's early work in Edinburgh concerned several branches of physics and chemistry. In 1867, he worked on the structures of various organic compounds and during his visit to Ghent may have suggested the structural formula for benzene that is credited to Kekulé. He also proposed formulas for pyridine and quinoline. In 1872, Dewar invented the vacuum flask as an insulating container in the course of an investigation of hydrogen-absorbed palladium. In collaboration with Peter Tait (1831-1901), he found that charcoal could be used as an absorbent to improve the strength of the vacuum. The vacuum prevented heat conduction or convection, so heat loss could occur only by radiation. The reflective silver layer on the walls of a vacuum flask are designed to minimize this.
When Dewar went to Cambridge in 1875, he began a long-term collaborative project on spectroscopy with George Liveing (1827-1924). They examined the correlation between spectral lines and bands and molecular states, and were particularly concerned with the absorption spectra of metals.
Dewar's move to the laboratories of the Royal Institution in London in 1877 marks the beginning of his main work on the liquefaction of gases and an examination of cryogenic properties. In that year, Louis Cailletet and Raoul Pictet (1846-1929) in France had succeeded in liquefying oxygen and nitrogen, two gases that had resisted Faraday's attempts at liquefaction. Dewar first turned to the production of large quantities of liquid oxygen, examining its properties. He found in 1891 that both liquid oxygen and ozone are magnetic. His vacuum flasks were invaluable in this work because they enabled him to preserve the very cold liquids for longer than would otherwise have been possible.
Dewar developed the cooling technique used in liquefaction by applying the Joule-Thomson effect, whereby expansion of a compressed gas results in a lowering of temperature due to the energy used in overcoming attractive forces between the gas molecules. By subjecting an already chilled gas to this process, he became the first to produce liquid hydrogen in 1895, though only in small quantities; then by using bigger apparatus he made larger amounts in 1898. He proceeded to examine the refractive index of liquid hydrogen, and a year later succeeded in solidifying hydrogen at a temperature of −259° C/−434°F.
Every gas had now been liquefied and solidified except one - helium. Dewar tried to liquefy helium, but was unsuccessful because his sample was contaminated with neon, which froze at the low temperature and blocked the apparatus. Heike Kamerlingh Onnes managed to liquefy helium using Dewar's techniques in 1908, and Dewar was then able to achieve temperatures within a degree of absolute zero (−273°C/−459°F) by boiling helium at low pressure.
Dewar investigated properties such as chemical reactivity, strength, and phosphorescence at low temperature. Substances such as feathers, for example, were found to be phosphorescent at these temperatures. From 1892-95, he studied with John Fleming (the inventor of the thermionic valve) the electric and magnetic properties of metals at low temperatures. They predicted that electrical resistance becomes negligible at extremely low temperatures, this phenomenon of superconductivity subsequently being detected by Kamerlingh Onnes in 1911.
During the World War I cryogenic research, being very expensive, had to be suspended. Dewar turned to the study of thin films, and later to the measurement of infrared radiation. He was a scientist with tremendous experimental flair and patient application.
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