Possible Clues:

• PV = k, to a physicist
• Pressure principle of physics
• Heat up salad -- it sounds like a bit of scientific theory
• Cook cabbage salad in hot water?
• Cook cabbage — it's said to be a scientific principle
• Confined gas at constant temperature varies inversely with pressure
• Truth about gas - child affected as well
• Gas pressure principle, by including oxygen as well, failing
• Be slowly cracking up about a principle involving pressure of 27