Siemens cycle
The Siemens cycle is a technique used to cool or liquefy gases. A gas is compressed, leading to an increase in its temperature due to the directly proportional relationship between temperature and pressure (as stated by Gay-Lussac's law). The compressed gas is then cooled by a heat exchanger and decompressed, resulting in a (possibly condensed) gas that is colder than the original at the same pressure. Carl Wilhelm Siemens patented the Siemens cycle in 1857. In the Siemens cycle the gas is: The gas which is now at its coolest in the current cycle, is recycled  and sent back to be - In each cycle the net cooling is more than the heat added at the beginning of the cycle. As the gas passes more cycles and becomes cooler, reaching lower temperatures at the expanding cylinder (stage 4 of the Siemens cycle) becomes more difficult.