Helium vessel. Container holding liquid helium, a substance used as a refrigerant in low temperature research; gaseous helium is also liquefied when large amounts are being transported. Helium liquefies at -268.9 degrees Celsius (C) and must be cooled to -272.2 C at a pressure of 25 atmospheres before it will solidify; these temperatures are only just above absolute zero (- 273.18 C). If liquid helium is cooled to -270.98 C it greatly changes its properties and is known as Helium II. Helium II has a high thermal conductivity, no internal temperature differences and zero viscosity. The main source of helium is natural gas, in which it is an impurity.