( ( Ca ( , ( Ti ( , ( and Al ) ) ) ) ) ( ( ( ( have ( ( similar depletions ) ( in ( low-density gas ) ) ) ) but ) ( ( have ( ( a ( steeper dependence ) ) ( of depletion ) ) ) ( on ( mean ( gas density ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Ca) (, ,) (NNP Ti) (, ,) (CC and) (NNP Al)) (VP (VP (VBP have) (NP (NP (JJ similar) (NNS depletions)) (PP (IN in) (NP (NN low-density) (NN gas))))) (CC but) (VP (VBP have) (NP (NP (DT a) (JJR steeper) (NN dependence)) (PP (IN of) (NP (NN depletion)))) (PP (IN on) (NP (JJ mean) (NN gas) (NN density))))) (. .)))
Ca, Ti, and Al have similar depletions in low-density gas but have a steeper dependence of depletion on mean gas density.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))