7.11 Properties of Liquid Helium
Helium was first obtained in liquid form by Kamerlingh Onnes in 1908 as a colourless, mobile and very volatile liquid boiling at 4.22 K under 1 atmosphere pressure. All attempts to solidify helium by boiling under reduced pressure failed; for even under a pressure of 0.013 mm of Hg (corresponding temperature 0.82 K), it remained in liquid state. Helium was first obtained in the solid state by Keesom in 1926 by the combined effects of high pressure and low temperature. He found that helium melts at 4 K under a pressure of 126 kg/cm2. According to Simon, melting point of helium is 42 K under a pressure of 5600 atmospheres. Evidently, liquid helium contracts on freezing. Critical temperature and pressure of liquid ...
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