If the atmospheric pressure is H cm of water, then, for the formation of the first bubble containing saturated water vapour, it is necessary that the saturated vapour pressure be not less than H—h cm of water. Therefore, at the beginning of the heating, when the water temperature is still low and the saturated vapour pressure is smaller than H — A, the water will fill the entire test-tube. At a temperature T < 100°C, which corresponds to a saturated vapour pressure of H—h, the water level breaks contact with the bottom of the test-tube. Under further heating, as the temperature and saturated vapour pressure increases, the level of the water in the test-tube drops, and at a temperature of 100°C it occupies the same level as the water in the glass.
