Quantum Mechanical Liquid: - Liquid helium is unique in that it exists in two forms He I and He II (liquid helium is obtained by joule-Thomson expansion of the gas previously cooled to 15K which is below the inversion temperature of He i.e., 35K). The liquid helium I boils at 4.2K and has normal liquid properties. One cooling he I to 2.19 K and 38 mm pressure, it changes to He II with abrupt changes in many physical properties such as density, dielectric constant and specific heat. He II is super fluid having so low energy that thermal motion of atoms do not take place, however, interatomic forces are also so weak that it does not occupy solid state. Thus He II is liquid with properties of gas. It has following characteristics:-
(i)          It has very high heat conductance -600 times of copper.
(ii)         It has very low viscosity – 1/100 of H2 gas.
(iii)        It has very flat meniscus – a low surface tension and creeps over the surface of glass container.