Cryogenics

The development of cryogenics as a low temperature science is a direct result of attempts by nineteenth century scientists to liquefy the permanent gases. One of these scientists, Michael Faraday, had succeeded, by 1845, in liquefying most of the gases then known to exist. His procedure consisted of cooling the gas by immersion in a bath of ether and dry ice and then pressurizing the gas until it liquefied. Six gases, however, resisted every attempt at liquefaction and were thus known at the time as permanent gases. They were oxygen, hydrogen, nitrogen, carbon monoxide, methane, and nitric oxide. The noble gases, helium, neon, argon, krypton, and xenon, not yet discovered.

Of the known permanent gases, oxygen and nitrogen, the primary constituents of air, received the most attention. For many years investigators labored to liquefy air. Finally, in 1877, Louis Cailletet in France and Raoul Pictet in Switzerland, succeeded in producing the first droplets of liquid air, and in 1883 the first measurable quantity of liquid oxygen was produced by S. F. von Wroblewski at the University of Cracow. Oxygen was found to liquefy at 90K (-297°F [-183°C]), and nitrogen at 77K (-320°F [-196°C]).

Following the liquefaction of air, a race to liquefy hydrogen ensued. James Dewar, a Scottish chemist, succeeded in 1898. He found the boiling point of hydrogen to be a frosty 20K (-423°F [-253°C]). In the same year, Dewar succeeded in freezing hydrogen, thus reaching the lowest temperature achieved to that time, 14K

(-434°F [-259°C]). Along the way, argon was discovered (1894) as an impurity in liquid nitrogen, and krypton and xenon were discovered (1898) during the fractional distillation of liquid argon. Fractional distillation is accomplished by liquefying a mixture of gases each of which has a different boiling point. When the mixture is evaporated, the gas with the highest boiling point evaporates first, followed by the gas with the second highest boiling point, and so on. Each of the newly discovered gases condensed at temperatures higher than the boiling point of hydrogen, but lower than 173K (-148°F [-100°C]).

The last element to be liquefied was helium gas. First discovered in 1868 in the spectrum of Sun, and later on Earth (1885), helium has the lowest boiling point of any known substance. In 1908, the Dutch physicist Heike Kamerlingh Onnes finally succeeded in liquefying helium at a temperature of 4.2K (-452°F).