( ( ( ( ( Eventually ( ( ( the density ) ( of atoms ) ) ( is ( ( low enough ) ( that ( ( not ( many photons ) ) ( find ( atoms ( to ( absorb them ) ) ) ) ) ) ) ) ) ) , ) and ) ( ( the photons ) ( become ( ( an ( independent part ) ) ( of ( the gas ) ) ) ) ) ) . )
(ROOT (S (S (ADVP (RB Eventually)) (NP (NP (DT the) (NN density)) (PP (IN of) (NP (NNS atoms)))) (VP (VBZ is) (ADJP (JJ low) (RB enough) (SBAR (IN that) (S (NP (RB not) (JJ many) (NNS photons)) (VP (VBP find) (S (NP (NNS atoms)) (VP (TO to) (VP (VB absorb) (NP (PRP them))))))))))) (, ,) (CC and) (S (NP (DT the) (NNS photons)) (VP (VBP become) (NP (NP (DT an) (JJ independent) (NN part)) (PP (IN of) (NP (DT the) (NN gas)))))) (. .)))
Eventually the density of atoms is low enough that not many photons find atoms to absorb them, and the photons become an independent part of the gas.
(ROOT (S (NP (NP (DT The) (JJ low) (JJ atomic) (NN number)) (PP (IN of) (NP (NN beryllium)))) (VP (VBZ allows) (S (NP (PRP it)) (VP (TO to) (VP (VB absorb) (NP (NNPS x-rays)))))) (. .)))