( ( In coals ) ( , ( however ( , ( ( these ( liquid hydrocarbons ) ) ( ( ( ( ( are commonly ) ( retained ( within ( ( the microstructure ) ( of ( the coal ) ) ) ) ) ) and ) ( ( are probably ) ( thermally ( ( cracked ( at ( higher temperatures ) ) ) ( to ( form gas ) ) ) ) ) ) . ) ) ) ) ) )
(ROOT (S (PP (IN In) (NP (NNS coals))) (, ,) (ADVP (RB however)) (, ,) (NP (DT these) (JJ liquid) (NNS hydrocarbons)) (VP (VP (VBP are) (ADVP (RB commonly)) (VP (VBN retained) (PP (IN within) (NP (NP (DT the) (NN microstructure)) (PP (IN of) (NP (DT the) (NN coal))))))) (CC and) (VP (VBP are) (ADVP (RB probably)) (VP (ADVP (RB thermally)) (VBN cracked) (PP (IN at) (NP (JJR higher) (NNS temperatures))) (S (VP (TO to) (VP (VB form) (NP (NN gas)))))))) (. .)))
In  coals, however, these liquid hydrocarbons are commonly retained within the microstructure of the coal and are probably thermally cracked at higher temperatures to form gas.
(ROOT (S (NP (PRP We)) (VP (VBP call) (NP (NP (DT the) (JJ solid) (NN form)) (PP (IN of) (NP (JJ hydrocarbons) (NN coal))))) (. .)))