cryogenics A branch of Physics which deals with the techniques of  production of low temperature, and the physical properties of solids and liquids at low temperatures.

Low  temperatures are obtained by liquefaction of gases. In Table CVI the boiling points of some of the gases are shown. The physical  principle  of  liquefaction  of gases is Joule Thomson effect*. The Joule Thomson Cooling in most gases is very small. Therefore regenerative process is employed, by which a portion of the cooled gas  which  has  already  undergone  Joule  Thomson expansion, is used to cool the incoming gas before latter suffers Joule  Thomson expansion. In this way cooling effect is rendered cumulative.  This principle is utilized for producing liquids of Nitrogen. Hydrogen and Helium, with modifications in the design of the machines.

Further lower temperatures (10^-2K) can only be achieved by the principle of adiabatic demagnetization.*

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                           Table - CVI