( ( ( cool gas ) ( at ( low density ) ) ) . )
(ROOT (S (VP (VB cool) (NP (NN gas)) (PP (IN at) (NP (JJ low) (NN density)))) (. .)))
cool gas at low density.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))