II.

Development of Cryogenics

Pioneering work in low-temperature physics by the British chemists Sir Humphry Davy and Michael Faraday, between 1823 and 1845, prepared the way for the development of cryogenics. Davy and Faraday generated gases by heating an appropriate mixture at one end of a sealed tube shaped like an inverted V. The other end was chilled in a salt-ice mixture. The combination of reduced temperature and increased pressure caused the evolved gas to liquefy. When the tube was opened, the liquid evaporated rapidly and cooled to its normal boiling point. By evaporating solid carbon dioxide mixed with ether, at low pressure, Faraday finally succeeded in reaching a temperature of about 163 K (about -110°C/-166°F).

The temperature of a gas that is allowed to expand can increase or decrease depending on the initial temperature of the gas. The special temperature at which a particular gas will cool down instead of heat up when it expands is called the inversion temperature. If a gas initially at a moderate temperature is expanded through a valve, its temperature increases. But if its initial temperature is below the inversion temperature, the expansion will cause a temperature reduction as the result of what is called the Joule-Thomson effect.

The inversion temperatures of hydrogen and helium, two primary cryogenic gases, are extremely low, and to achieve a temperature reduction through expansion, these gases must first be precooled below their inversion temperatures, the hydrogen by liquid air and the helium by liquid hydrogen. This method is generally not able to bring about liquefaction in one step, but by cascading the effects, the French physicist Louis Paul Cailletet and the Swiss scientist Raoul Pierre Pictet were able in 1877 to produce droplets of liquid oxygen. The success of these experimenters marked the end of the idea of permanent gases and established the possibility of liquefying any gas by moderate compression at temperatures below the critical temperature.

The Dutch physicist Heike Kamerlingh Onnes set up the first liquid-air plant in 1894, using the cascade principle. Investigators in Britain, France, Germany, and Russia developed various improvements in the process during the following 40 years. The British chemist Sir James Dewar first liquefied hydrogen in 1898 and Kamerlingh Onnes liquefied helium, the most difficult of the gases to liquefy, in 1908.

The increased efficiency of having a refrigerant gas operate in a reciprocating engine or in a turbine continued to be a challenge. The work of the Soviet physicist Peter Leonidovich Kapitza and the American mechanical engineer Samuel Collins led to better ways to study phenomena at low temperatures. A helium-liquefier based on Collins’s design provided the opportunity for many nonspecialist laboratories to conduct experiments at the normal boiling point of helium, 4.2 K (-268.9°C/-452.0°F).