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the cylinder (known as Bottom Dead Centre or BDC) the exhaust valve starts to open. As the piston now moves up the cylinder, the hot gases (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen) are expelled from the cylinder. As the Piston approaches TDC again the inlet valve starts to open and the cycle repeats itself.
Volcanic Gas. During an eruption of a volcano, gases dissolved in the magma are released into the air. These gases can escape through many different places in the volcano, such as the large opening at the top or vents in the side.