<P> Liquid helium - 4 and the rare helium - 3 are not completely miscible . Below 0.9 kelvin at their saturated vapor pressure, a mixture of the two isotopes undergoes a phase separation into a normal fluid (mostly helium - 3) that floats on a denser superfluid consisting mostly of helium - 4 . This phase separation happens because the overall mass of liquid helium can reduce its thermodynamic enthalpy by separating . </P> <P> At extremely low temperatures, the superfluid phase, rich in helium - 4, can contain up to 6% of helium - 3 in solution . This makes possible the small - scale use of the dilution refrigerator, which is capable of reaching temperatures of a few millikelvins . </P> <P> Superfluid helium - 4 has substantially different properties from ordinary liquid helium . </P> <Table> <Tr> <Th> Properties of liquid helium </Th> <Th> Helium - 4 </Th> <Th> Helium - 3 </Th> </Tr> <Tr> <Td> Critical temperature </Td> <Td> 5.2 K </Td> <Td> 3.3 K </Td> </Tr> <Tr> <Td> Boiling point at one atmosphere </Td> <Td> 4.2 K </Td> <Td> 3.2 K </Td> </Tr> <Tr> <Td> Minimum melting pressure </Td> <Td> 25 atm </Td> <Td> 29 atm at 0.3 K </Td> </Tr> <Tr> <Td> Superfluid transition temperature at saturated vapor pressure </Td> <Td> 2.17 K </Td> <Td> 1 mK in the absence of a magnetic field </Td> </Tr> </Table>

The noble gas which behaves abnormally in liquid state