====== Normal 4He properties ======

  * The Observed Properties of Liquid Helium at the Saturated Vapor Pressure. R. J. Donnelly and C. F. Barenghi, Journal of Physical and Chemical Reference Data 27, 1217 (1998) https://doi.org/10.1063/1.556028. These properties are available interactively online at http://www.mas.ncl.ac.uk/helium/
  * NIST site https://trc.nist.gov/cryogenics/materials/materialproperties.htm