Most homes possess a Thermos flask, used for keeping tea or coffee hot.  The flask, by providing the very best kind of insulation – a vacuum – around the liquid, keeps it warm even after several hours.  The strange fact is, however, that this kind of flask was originally designed, not to keep liquids warm at all, but to keep them cold.  The insulation ensures that it does both equally efficiently.

Sir James Dewar, the inventor of the first vacuum flask, was a noted Scottish chemist.  Born at Kincardine in 1842, James Dewar trained both in Britain and on the Continent and finally became a professor at Cambridge and at the Royal Institution in London.  He became interested in turning gases into liquids at low temperatures.  In 1898, for example, he succeeded in liquefying hydrogen by cooling and compressing it.  In the course of his experiments the chemist found the need for a device to keep liquid air cool and prevent it from boiling away.  Since liquid air begins to turn back into gas at about minus 200?C., there was a very considerable problem in storing it.

The flask which James Dewar devised for the purpose was quite a simple affair.  It was merely a double-walled glass vessel.  All the air was drawn out of the space between the two walls by a pump, so that any liquid in the flask was surrounded, to all intents and purposes, by a vacuum.  The most important way in which the liquid would normally gain heat – by conduction through the surrounding air – was therefore eliminated.  (Heat in the form of waves – heat radiation – is not stopped by a vacuum.  Later flasks had their walls silvered to cut down the amount of heat radiation.)

Sir James Dewar also contributed to the discovery of cordite, now important in explosives.  James Dewar died in 1923, having received, amongst other honours, the Rumford Medal of the Royal Society.

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