Thermal expansion of gas. Experiment showing how gas expands with increasing temperature, a phenomenon known as Charles' Law. The experiment consists of a measuring cylinder (center) standing upside down in a beaker of water. The measuring cylinder contains trapped air; the level of the water within the cylinder is shown by a floating yellow ball. A yellow thermometer (at left) measures the temperature of the water. As the water is heated the air will expand and force the water further down the measuring cylinder. Charles' Law states that 'For a fixed mass of gas at a constant pressure, the volume is directly proportional to its absolute temperature'.