Complex sentence: If the volume of two gases which are kept at constant temperature and pressure below critical conditions is gradually reduced condensation will start when a certain volume is reached, the amount of condensation will gradually increase upon further reduction in volume until the gases are liquefied.
Split into simpler sentences: If the volume of two gases which are kept at constant temperature and pressure below critical conditions is gradually reduced, condensation will start. When a certain volume is reached, the amount of condensation will gradually increase upon further reduction in volume until the gases are liquefied.