Sentence:  When values of temperature derived with this instrument were compared with the accepted values associated with liquid <WSD>helium</WSD> 4 vapor pressures , differences of about 10 and 7 millidegrees respectively were found . 
Meaning of the word marked with <WSD> tags:  a very light colorless element that is one of the six inert gasses; the most difficult gas to liquefy; occurs in economically extractable amounts in certain natural gases (as those found in Texas and Kansas)