Demonstration. - Place a quantity of water in the flask A (Fig. 1). Raise the water to the boiling point, and after it has boiled a short time remove the flame
and put in a rubber stopper with tube, as shown in the figure. Replace the flame, and the steam formed by boiling will collect in the flask above the surface of the water and increase the pressure. As this pressure increases it will do work by forcing water out of the tube.
In the process of evaporation the molecules, moving in every direction, strike the surface of the liquid from below, and some of them escape into the air. If the temperature is raised, the velocity of the molecules is increased, and a greater number escape. When boiling begins, at 100° under ordinary atmospheric pressure, the rise of temperature is stopped, and all the heat energy applied is used in changing water to steam. If the boiling takes place in a closed vessel, the repeated blows of the molecules of the vapor upon the surface of the liquid oppose the escaping molecules more and more as the vapor increases in quantity and pressure; and if the temperature is kept at any fixed point, as 100°, the boiling soon stops. If the temperature is raised, the molecular velocity increases, the internal pressure becomes greater than the vapor pressure, and boiling recommences.
