Carbon dioxide is sealed inside a glass tube. Below 31°C, CO₂ is in a liquid state under high pressure. When the temperature rises above 31°C, CO₂ can no longer be liquefied even at very high pressure and instead enters the supercritical state. Therefore, above 31°C you can observe the fourth state of matter: the supercritical state (https://en.wikipedia.org/wiki/Supercritical_fluid).
