Abstract: The development of liquefaction has experienced a long and tortuous process. Helium was liquefied 100 years ago, so 2008 was a historical year. By looking back the history of liquefaction, we may know the cognition of knowledge is a dialectical process. This article is for commemorating the brilliant scientists Sir James Dewar and Kamerlingh Onnes for their outstanding contribution in the liquefaction

Key words: liquefaction, dialectic process, temperature measurement, air separation