Sentence:  Surface temperature gradients have been found to exist in liquid <WSD>helium</WSD> baths contained in 15 - and 25 - liter metallic storage dewars . 
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)