They are sealed by the pressure of the gas , as the pressure rises , a slackly threaded cap presses up against a sealing ring which is screwed down with another cap , thus making an airtight seal .
PRON AUX VERB ADP DET NOUN ADP DET NOUN , ADP DET NOUN VERB , DET ADJ VERB NOUN VERB ADP ADP DET NOUN NOUN DET AUX VERB ADP ADP DET NOUN , ADV VERB DET NOUN NOUN .