( It ( ( ( had long ) ( been ( recognized ( that ( ( in ( ( ( a ( neutral gas ) ) , ) ( such ( as ( ( the ( earth 's ) ) atmosphere ) ) ) ) ) ( , ( particles ( must ( collide ( if ( shocks ( are ( to form ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBD had) (ADVP (RB long)) (VP (VBN been) (VP (VBN recognized) (SBAR (IN that) (S (PP (IN in) (NP (NP (DT a) (JJ neutral) (NN gas)) (, ,) (PP (JJ such) (IN as) (NP (NP (DT the) (NN earth) (POS 's)) (NN atmosphere))))) (, ,) (NP (NNS particles)) (VP (MD must) (VP (VB collide) (SBAR (IN if) (S (NP (NNS shocks)) (VP (VBP are) (S (VP (TO to) (VP (VB form)))))))))))))) (. .)))
It had long been recognized that in a neutral gas, such as the earth's atmosphere, particles must collide if shocks are to form.
(ROOT (S (NP (JJ Volcanic) (NNS gases)) (VP (VBP helpe) (S (VP (TO to) (VP (VB form) (NP (DT the) (NN atmosphere)) (PP (IN on) (NP (NN earth))))))) (. .)))