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By 1908 , James Dewar and Heike Kamerlingh Onnes were successfully able to liquefy hydrogen and then newly discovered helium , respectively .
 James Dewar, Heike Kamerlingh Onnes
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 James Dewar, Heike Kamerlingh Onnes