( ( A ( Knudsen gas ) ) ( ( ( is ( ( a gas ) ( in ( ( a state ) ( of ( such ( low density ) ) ) ) ) ) ) ( that ( ( ( the ( mean ( free path ) ) ) ( of molecules ) ) ( is ( greater ( than ( ( ( the diameter ) ( of receptacle ) ) ( that ( contains it ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NNP Knudsen) (NN gas)) (VP (VBZ is) (NP (NP (DT a) (NN gas)) (PP (IN in) (NP (NP (DT a) (NN state)) (PP (IN of) (NP (JJ such) (JJ low) (NN density)))))) (SBAR (IN that) (S (NP (NP (DT the) (JJ mean) (JJ free) (NN path)) (PP (IN of) (NP (NNS molecules)))) (VP (VBZ is) (ADJP (ADJP (JJR greater)) (PP (IN than) (NP (NP (DT the) (NN diameter)) (PP (IN of) (NP (NN receptacle))) (SBAR (WHNP (WDT that)) (S (VP (VBZ contains) (NP (PRP it)))))))))))) (. .)))
A Knudsen gas is a gas in a state of such low density that the mean free path of molecules is greater than the diameter of receptacle that contains it.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))