Significant work was done in the early nineteenth century by Michael Faraday at the Royal Institution in London. Faraday was an apprentice bookbinder from a poor family. He managed to secure a job with the Royal Institution as a scientific assistant with the eminent chemist Sir Humphrey Davy, mainly on the strength of presenting him with a bound version of notes he had taken at some of Davy's public lectures. Davy's wife still treated him like a servant. He accidentally discovered a way to make liquid chlorine.
In 1811, Davy had showed that the crystals obtained by passing chlorine gas through a nearly freezing dilute solution of calcium chloride were a compound of chlorine and water; chlorine hydrate (Cl2H2O). At Davy's suggestion, Faraday performed some experiments. He heated a sealed glass tube containing the above crystals. The other end of the tube was submerged in ice. An oily liquid condensed at the cold end which, experiments showed, was chlorine. The high pressure in the tube allowed the liquefaction to take place at a lower temperature (-34 C) than it would at ambient pressure. Davy was the first to liquefy an element though compounds had been liquefied earlier. This technique was then used to liquefy other gases like ammonia, hydrogen sulfide, nitrogen dioxide etc. Faraday called hydrogen, nitrogen, and oxygen permanent gases since he was unable to liquefy them. Actually, it was the limitation on obtaining sufficient pressure which prevented him from liquefying the other gases. Another original procedure for cooling and liquefying gases was discovered by accident. Louis Paul Cailletet (son of a metallurgist) had set up a lab in his father's foundry. He was trying to liquefy acetylene. During the pressurisation, his tube sprang a leak. As the gas escaped through the leak, a faint mist was formed near the outlet which quickly disappeared. The first instinct was to suspect water impurity in the starting material. But this also happened in pure samples. Another way to obtain lower temperatures was therefore to release the pressure instead of augmenting it. Thus was born a new scheme to liquefy gases. He then carried out the process (reported in the academy of science in Paris in 1877) for oxygen successfully. He further went on to liquefy nitrogen and carbon monoxide. Another method was developed by swiss chemist Raoul-Peirre Pictet in Geneva based on a cascade process.