( ( The bubbles ) ( ( will ( ( descend and ) ( then ( ( float over ) ( ( the ( denser layer ) ) ( of ( carbon ( dioxide gas ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS bubbles)) (VP (MD will) (VP (VP (VB descend)) (CC and) (VP (ADVP (RB then)) (VB float) (PRT (RP over)) (NP (NP (DT the) (NN denser) (NN layer)) (PP (IN of) (NP (NN carbon) (NN dioxide) (NN gas))))))) (. .)))
The bubbles will descend and then float over the denser layer of carbon dioxide gas.
(ROOT (S (NP (NNP Gas) (NN bubble) (NNS holes)) (VP (VBZ helps) (NP (NP (JJ pumice) (NN float)) (PP (IN in) (NP (NN water))))) (. .)))