Liquefaction of Gases — the Basic Principles
Liquefaction of gases is an exercise which had attracted the attention of scientists from very early times. As we know, increased pressure and lowering of temperature are used together or singly for the purpose. In many cases the investigators succeeded in liquefying easily liquefaction gases like NH3, SO2, Cl2, HCl etc. Notable among the early work is due to Faraday (1823) who used a very simple device and was able to liquefy SO2, H2S, CO2, N2O and NH3. He used an inverted U-tube (Figure) in one limb of which the gas was prepared from suitable reagents and in the other the gas was liquefied under its own pressure with the help of cooling by immersion in a freezing mixture of ice and salt.
Thilorier (1834) used Faraday’s apparatus, but made of wrought iron instead of glass, to liquefy CO2 in large quantities, and even to obtain solid CO2 by allowing partial evaporation of the liquid. He obtained the lowest temperature (-1100 C) stainable at the time by using a freezing mixture of solid CO2 and ether. The so-called permanent gases were not liquefied until Andrews discovered the critical phenomenon. His conclusion was that all gases could be liquefied provided these were cooled.
Below their respective crucial temperature; but as long as the temperature was above the critical value no amount of pressure would bring the gas to the liquid state.
Obviously gases like H2, N2, O2 etc. had critical temperatures lower than the lowest temperature attainable at the time. Thus followed an intense attempt at attaining very low temperature by the application of different physical principles.